diff --git a/binaries/data/mods/_test.gui/gui/settings/cguisize/assign.js b/binaries/data/mods/_test.gui/gui/settings/cguisize/assign.js
new file mode 100644
index 0000000000..0ed45ed31d
--- /dev/null
+++ b/binaries/data/mods/_test.gui/gui/settings/cguisize/assign.js
@@ -0,0 +1,4 @@
+testObject.size = {
+	"left": 3,
+	"bottom": 2
+};
diff --git a/binaries/data/mods/_test.gui/gui/settings/cguisize/compoundassignmentoperator.js b/binaries/data/mods/_test.gui/gui/settings/cguisize/compoundassignmentoperator.js
new file mode 100644
index 0000000000..d721f2cd8f
--- /dev/null
+++ b/binaries/data/mods/_test.gui/gui/settings/cguisize/compoundassignmentoperator.js
@@ -0,0 +1 @@
+testObject.size.left += 5;
diff --git a/binaries/data/mods/_test.gui/gui/settings/cguisize/lazyassign.js b/binaries/data/mods/_test.gui/gui/settings/cguisize/lazyassign.js
new file mode 100644
index 0000000000..f984d29c05
--- /dev/null
+++ b/binaries/data/mods/_test.gui/gui/settings/cguisize/lazyassign.js
@@ -0,0 +1 @@
+testObject.size.left = 5;
diff --git a/binaries/data/mods/_test.gui/gui/settings/cguisize/objectassign.js b/binaries/data/mods/_test.gui/gui/settings/cguisize/objectassign.js
new file mode 100644
index 0000000000..3e03485123
--- /dev/null
+++ b/binaries/data/mods/_test.gui/gui/settings/cguisize/objectassign.js
@@ -0,0 +1,3 @@
+Object.assign(testObject.size, {
+	"rleft": 4, "rbottom": 20
+});
diff --git a/source/gui/CGUI.cpp b/source/gui/CGUI.cpp
index 195bb0fdab..ac550d4d01 100644
--- a/source/gui/CGUI.cpp
+++ b/source/gui/CGUI.cpp
@@ -1,4 +1,4 @@
-/* Copyright (C) 2024 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -279,6 +279,7 @@ InReaction CGUI::HandleEvent(const SDL_Event_* ev)
 
 void CGUI::TickObjects()
 {
+	m_BaseObject->RecurseObject(&IGUIObject::IsHidden, &IGUIObject::DispatchDelayedSettingChanges);
 	m_BaseObject->RecurseObject(&IGUIObject::IsHiddenOrGhostOrOutOfBoundaries, &IGUIObject::Tick);
 	SendEventToAll(EventNameTick);
 	m_Tooltip.Update(FindObjectUnderMouse(), m_MousePos, *this);
diff --git a/source/gui/CGUISetting.cpp b/source/gui/CGUISetting.cpp
index e1ff781ff0..fe97e6ad58 100644
--- a/source/gui/CGUISetting.cpp
+++ b/source/gui/CGUISetting.cpp
@@ -119,7 +119,6 @@ TYPE(CVector2D)
 TYPE(CStr)
 TYPE(CStrW)
 // TODO: make these inherit from CGUISimpleSetting directly.
-TYPE(CGUISize)
 TYPE(CGUIColor)
 TYPE(CGUISpriteInstance)
 TYPE(CGUIString)
diff --git a/source/gui/CGUISetting.h b/source/gui/CGUISetting.h
index 82ebcdccc4..252800bf14 100644
--- a/source/gui/CGUISetting.h
+++ b/source/gui/CGUISetting.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2023 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -50,6 +50,18 @@ public:
 	 */
 	virtual void ToJSVal(const ScriptRequest& rq, JS::MutableHandleValue value) = 0;
 
+	void DeferSettingChange()
+	{
+		delayNotification = true;
+	}
+
+	void DispatchDelayedSettingChange()
+	{
+		if (!std::exchange(delayNotification, false))
+			return;
+
+		OnSettingChange(GetName(), true);
+	}
 protected:
 	IGUISetting(IGUISetting&& other);
 	IGUISetting& operator=(IGUISetting&& other) = delete;
@@ -79,6 +91,7 @@ protected:
 	 */
 	IGUIObject& m_Object;
 
+	bool delayNotification{false};
 private:
 	CStr m_Name;
 };
diff --git a/source/gui/ObjectBases/IGUIObject.cpp b/source/gui/ObjectBases/IGUIObject.cpp
index 24cbfc9022..511f63cc02 100644
--- a/source/gui/ObjectBases/IGUIObject.cpp
+++ b/source/gui/ObjectBases/IGUIObject.cpp
@@ -1,4 +1,4 @@
-/* Copyright (C) 2024 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -256,6 +256,8 @@ void IGUIObject::UpdateCachedSize()
 
 CRect IGUIObject::GetComputedSize()
 {
+	// Ensure the size is up to date before we use it.
+	m_Settings.at("size")->DispatchDelayedSettingChange();
 	UpdateCachedSize();
 	return m_CachedActualSize;
 }
@@ -564,3 +566,9 @@ void IGUIObject::DrawInArea(CCanvas2D& canvas, CRect& area)
 bool IGUIObject::IsHiddenOrGhostOrOutOfBoundaries() const {
 	return !m_IsInsideBoundaries || IsHiddenOrGhost();
 }
+
+void IGUIObject::DispatchDelayedSettingChanges()
+{
+	for (const auto& setting : m_Settings)
+		setting.second->DispatchDelayedSettingChange();
+}
diff --git a/source/gui/ObjectBases/IGUIObject.h b/source/gui/ObjectBases/IGUIObject.h
index 380b858bbd..17a9bff851 100644
--- a/source/gui/ObjectBases/IGUIObject.h
+++ b/source/gui/ObjectBases/IGUIObject.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2024 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -271,6 +271,8 @@ public:
 
 	virtual void DrawInArea(CCanvas2D& canvas, CRect& area);
 
+	virtual void DispatchDelayedSettingChanges();
+
 protected:
 	/**
 	 * Draws the object.
diff --git a/source/gui/ObjectBases/IGUIPanel.cpp b/source/gui/ObjectBases/IGUIPanel.cpp
index 3ab79c64b7..edb804a570 100644
--- a/source/gui/ObjectBases/IGUIPanel.cpp
+++ b/source/gui/ObjectBases/IGUIPanel.cpp
@@ -1,4 +1,4 @@
-/* Copyright (C) 20244 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -43,6 +43,8 @@ void IGUIPanel::UpdateCachedSize()
 
 CRect IGUIPanel::GetComputedSize()
 {
+	// Ensure the size is up to date before we use it.
+	m_Settings.at("size")->DispatchDelayedSettingChange();
 	UpdateCachedSize();
 	return m_CachedLayoutActualSize;
 }
diff --git a/source/gui/Scripting/GuiScriptConversions.cpp b/source/gui/Scripting/GuiScriptConversions.cpp
index 504d07b4f9..3098efcc39 100644
--- a/source/gui/Scripting/GuiScriptConversions.cpp
+++ b/source/gui/Scripting/GuiScriptConversions.cpp
@@ -1,4 +1,4 @@
-/* Copyright (C) 2024 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -22,7 +22,6 @@
 #include "gui/SettingTypes/CGUIColor.h"
 #include "gui/SettingTypes/CGUIList.h"
 #include "gui/SettingTypes/CGUISeries.h"
-#include "gui/SettingTypes/CGUISize.h"
 #include "gui/Scripting/JSInterface_GUIProxy.h"
 #include "lib/external_libraries/libsdl.h"
 #include "maths/Size2D.h"
@@ -194,16 +193,6 @@ template<> void Script::ToJSVal<CRect>(const ScriptRequest& rq, JS::MutableHandl
 		"bottom", val.bottom);
 }
 
-template<> void Script::ToJSVal<CGUISize>(const ScriptRequest& rq, JS::MutableHandleValue ret, const CGUISize& val)
-{
-	val.ToJSVal(rq, ret);
-}
-
-template<> bool Script::FromJSVal<CGUISize>(const ScriptRequest& rq, JS::HandleValue v, CGUISize& out)
-{
-	return out.FromJSVal(rq, v);
-}
-
 template<> void Script::ToJSVal<CGUIList>(const ScriptRequest& rq, JS::MutableHandleValue ret, const CGUIList& val)
 {
 	ToJSVal(rq, ret, val.m_Items);
diff --git a/source/gui/Scripting/JSInterface_CGUISize.cpp b/source/gui/Scripting/JSInterface_CGUISize.cpp
new file mode 100644
index 0000000000..734e39b949
--- /dev/null
+++ b/source/gui/Scripting/JSInterface_CGUISize.cpp
@@ -0,0 +1,216 @@
+/* Copyright (C) 2025 Wildfire Games.
+ * This file is part of 0 A.D.
+ *
+ * 0 A.D. is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 0 A.D. is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "precompiled.h"
+
+#include "JSInterface_CGUISize.h"
+
+#include "ps/CStr.h"
+#include "gui/CGUI.h"
+#include "gui/CGUISetting.h"
+#include "gui/ObjectBases/IGUIObject.h"
+#include "gui/SettingTypes/CGUISize.h"
+#include "gui/Scripting/JSInterface_GUISize.h"
+#include "maths/Rect.h"
+#include "scriptinterface/Object.h"
+#include "scriptinterface/ScriptInterface.h"
+#include "scriptinterface/ScriptForward.h"
+#include "scriptinterface/ScriptTypes.h"
+#include <fmt/format.h>
+
+namespace
+{
+template <float CRect::*Member, CRect CGUISize::*RectMember>
+bool GetCRectField(JSContext* cx, unsigned argc, JS::Value* vp)
+{
+	JS::CallArgs args{JS::CallArgsFromVp(argc, vp)};
+	JS::RootedObject obj{cx, &args.thisv().toObject()};
+	CGUISimpleSetting<CGUISize>* wrapper{JS::GetMaybePtrFromReservedSlot<CGUISimpleSetting<CGUISize>>(obj, ScriptInterface::JSObjectReservedSlots::PRIVATE)};
+
+	args.rval().setDouble(wrapper->GetMutable().*RectMember.*Member);
+	return true;
+}
+
+template <float CRect::*Member, CRect CGUISize::*RectMember>
+bool SetCRectField(JSContext* cx, unsigned argc, JS::Value* vp)
+{
+	JS::CallArgs args{JS::CallArgsFromVp(argc, vp)};
+	JS::RootedObject obj{cx, &args.thisv().toObject()};
+	CGUISimpleSetting<CGUISize>* wrapper{JS::GetMaybePtrFromReservedSlot<CGUISimpleSetting<CGUISize>>(obj, ScriptInterface::JSObjectReservedSlots::PRIVATE)};
+
+	double val;
+	if (!JS::ToNumber(cx, args.get(0), &val))
+		return false;
+
+	wrapper->GetMutable().*RectMember.*Member = static_cast<float>(val);
+	args.rval().setUndefined();
+
+	wrapper->DeferSettingChange();
+	return true;
+}
+
+// Produces "10", "-10", "50%", "50%-10", "50%+10", etc
+CStr ToPercentString(double pix, double per)
+{
+	if (per == 0)
+		return CStr::FromDouble(pix);
+
+	if (pix == 0)
+		return fmt::format("{}%", per);
+
+	return fmt::format("{}%{:+}", per, pix);
+}
+
+bool toString(JSContext* cx, uint argc, JS::Value* vp)
+{
+	JS::CallArgs args{JS::CallArgsFromVp(argc, vp)};
+	JS::RootedObject obj{cx, &args.thisv().toObject()};
+	CGUISimpleSetting<CGUISize>* wrapper{JS::GetMaybePtrFromReservedSlot<CGUISimpleSetting<CGUISize>>(obj, ScriptInterface::JSObjectReservedSlots::PRIVATE)};
+	CStr buffer;
+
+	buffer += ToPercentString(wrapper->GetMutable().pixel.left, wrapper->GetMutable().percent.left) + " ";
+	buffer += ToPercentString(wrapper->GetMutable().pixel.top, wrapper->GetMutable().percent.top) + " ";
+	buffer += ToPercentString(wrapper->GetMutable().pixel.right, wrapper->GetMutable().percent.right) + " ";
+	buffer += ToPercentString(wrapper->GetMutable().pixel.bottom, wrapper->GetMutable().percent.bottom);
+
+	ScriptRequest rq{cx};
+	Script::ToJSVal(rq, args.rval(), buffer);
+	return true;
+}
+
+JSClass JSI_class = {
+	"CGUISize", JSCLASS_HAS_RESERVED_SLOTS(1)
+};
+
+JSFunctionSpec JSI_methods[] =
+{
+	JS_FN("toString", toString, 0, 0),
+	JS_FN("toSource", toString, 0, 0),
+	JS_FS_END
+};
+
+JSPropertySpec JSI_props[] =
+{
+	JS_PSGS("top", (GetCRectField<&CRect::top, &CGUISize::pixel>), (SetCRectField<&CRect::top, &CGUISize::pixel>), JSPROP_ENUMERATE),
+	JS_PSGS("left", (GetCRectField<&CRect::left, &CGUISize::pixel>), (SetCRectField<&CRect::left, &CGUISize::pixel>), JSPROP_ENUMERATE),
+	JS_PSGS("bottom", (GetCRectField<&CRect::bottom, &CGUISize::pixel>), (SetCRectField<&CRect::bottom, &CGUISize::pixel>), JSPROP_ENUMERATE),
+	JS_PSGS("right", (GetCRectField<&CRect::right, &CGUISize::pixel>), (SetCRectField<&CRect::right, &CGUISize::pixel>), JSPROP_ENUMERATE),
+
+	JS_PSGS("rtop", (GetCRectField<&CRect::top, &CGUISize::percent>), (SetCRectField<&CRect::top, &CGUISize::percent>), JSPROP_ENUMERATE),
+	JS_PSGS("rleft", (GetCRectField<&CRect::left, &CGUISize::percent>), (SetCRectField<&CRect::left, &CGUISize::percent>), JSPROP_ENUMERATE),
+	JS_PSGS("rbottom", (GetCRectField<&CRect::bottom, &CGUISize::percent>), (SetCRectField<&CRect::bottom, &CGUISize::percent>), JSPROP_ENUMERATE),
+	JS_PSGS("rright", (GetCRectField<&CRect::right, &CGUISize::percent>), (SetCRectField<&CRect::right, &CGUISize::percent>), JSPROP_ENUMERATE),
+	JS_PS_END
+};
+} // end anonymous namespace
+
+
+
+void JSI_CGUISize::RegisterScriptClass(ScriptInterface& scriptInterface)
+{
+	scriptInterface.DefineCustomObjectType(&JSI_class, nullptr, 0, JSI_props, JSI_methods, nullptr, nullptr);
+}
+
+template class CGUISimpleSetting<CGUISize>;
+template<>
+void CGUISimpleSetting<CGUISize>::ToJSVal(const ScriptRequest& rq, JS::MutableHandleValue ret)
+{
+	const ScriptInterface& scriptInterface = rq.GetScriptInterface();
+	JS::RootedObject obj{rq.cx, scriptInterface.CreateCustomObject("CGUISize")};
+
+	JS::SetReservedSlot(obj, ScriptInterface::JSObjectReservedSlots::PRIVATE, JS::PrivateValue(this));
+	ret.setObject(*obj);
+};
+
+template<>
+bool CGUISimpleSetting<CGUISize>::DoFromString(const CStrW& value)
+{
+	return CGUI::ParseString<CGUISize>(&m_Object.GetGUI(), value, m_Setting);
+};
+
+template<>
+bool CGUISimpleSetting<CGUISize>::DoFromJSVal(const ScriptRequest& rq, JS::HandleValue value)
+{
+	if (value.isString())
+	{
+		CStrW str;
+		if (!Script::FromJSVal(rq, value, str))
+		{
+			LOGERROR("CGUISize could not read JS string");
+			return false;
+		}
+
+		if (!m_Setting.FromString(str.ToUTF8()))
+		{
+			LOGERROR("CGUISize could not parse JS string");
+			return false;
+		}
+
+		return true;
+	}
+
+	if (!value.isObject())
+	{
+		LOGERROR("CGUISize value is not an String, nor Object");
+		return false;
+	}
+
+	JS::RootedObject obj{rq.cx, &value.toObject()};
+	if (JS_InstanceOf(rq.cx, obj, &JSI_class, nullptr))
+	{
+		CGUISimpleSetting<CGUISize>* wrapper = JS::GetMaybePtrFromReservedSlot<CGUISimpleSetting<CGUISize>>(obj, ScriptInterface::JSObjectReservedSlots::PRIVATE);
+		if (this != wrapper)
+			this->Set(wrapper->m_Setting, false);
+		return true;
+	}
+
+	if (JS_InstanceOf(rq.cx, obj, &JSI_GUISize::JSI_class, nullptr))
+		LOGWARNING("Assigning an GUISize to CGUISize is deprecated. Please use the object.size = {left:number, top:number, right:number, bottom:number} format instead. This support will be removed in a future version.");
+
+	bool atLeastOnePropertySet{false};
+	CRect pixel{};
+	CRect percent{};
+	auto tryGet = [&](const char* name, float& field)
+	{
+		if (!Script::HasProperty(rq, value, name) || !Script::GetProperty(rq, value, name, field))
+			return;
+
+		atLeastOnePropertySet = true;
+	};
+
+	tryGet("left", pixel.left);
+	tryGet("top", pixel.top);
+	tryGet("right", pixel.right);
+	tryGet("bottom", pixel.bottom);
+
+	tryGet("rtop", percent.top);
+	tryGet("rleft", percent.left);
+	tryGet("rbottom", percent.bottom);
+	tryGet("rright", percent.right);
+
+	// If no properties were set, return false to indicate failure.
+	if (!atLeastOnePropertySet)
+	{
+		LOGERROR("CGUISize could not read JS object");
+		return false;
+	}
+
+	m_Setting.pixel = pixel;
+	m_Setting.percent = percent;
+
+	return true;
+};
diff --git a/source/gui/Scripting/JSInterface_CGUISize.h b/source/gui/Scripting/JSInterface_CGUISize.h
new file mode 100644
index 0000000000..98d0494526
--- /dev/null
+++ b/source/gui/Scripting/JSInterface_CGUISize.h
@@ -0,0 +1,27 @@
+/* Copyright (C) 2025 Wildfire Games.
+ * This file is part of 0 A.D.
+ *
+ * 0 A.D. is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 0 A.D. is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef INCLUDED_JSI_CGUISIZE
+#define INCLUDED_JSI_CGUISIZE
+
+class ScriptInterface;
+namespace JSI_CGUISize
+{
+	void RegisterScriptClass(ScriptInterface& scriptInterface);
+}
+
+#endif // INCLUDED_JSI_GUISIZE
diff --git a/source/gui/Scripting/JSInterface_GUIProxy_impl.h b/source/gui/Scripting/JSInterface_GUIProxy_impl.h
index ab3142742a..169bc2b450 100644
--- a/source/gui/Scripting/JSInterface_GUIProxy_impl.h
+++ b/source/gui/Scripting/JSInterface_GUIProxy_impl.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2023 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -31,6 +31,9 @@
 #include <string>
 #include <string_view>
 
+#ifndef INCLUDED_JSI_GUIPROXY_IMP
+#define INCLUDED_JSI_GUIPROXY_IMP
+
 template <typename T>
 JSI_GUIProxy<T>& JSI_GUIProxy<T>::Singleton()
 {
@@ -319,3 +322,4 @@ bool JSI_GUIProxy<T>::delete_(JSContext* cx, JS::HandleObject proxy, JS::HandleI
 	LOGERROR("Only event handlers can be deleted from GUI objects!");
 	return result.fail(JSMSG_BAD_PROP_ID);
 }
+#endif // INCLUDED_JSI_GUIPROXY_IMP
diff --git a/source/gui/Scripting/ScriptFunctions.cpp b/source/gui/Scripting/ScriptFunctions.cpp
index 472bf59fc1..18bb38a9cf 100644
--- a/source/gui/Scripting/ScriptFunctions.cpp
+++ b/source/gui/Scripting/ScriptFunctions.cpp
@@ -1,4 +1,4 @@
-/* Copyright (C) 2022 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -20,6 +20,7 @@
 #include "ScriptFunctions.h"
 
 #include "graphics/scripting/JSInterface_GameView.h"
+#include "gui/Scripting/JSInterface_CGUISize.h"
 #include "gui/Scripting/JSInterface_GUIManager.h"
 #include "gui/Scripting/JSInterface_GUISize.h"
 #include "i18n/scripting/JSInterface_L10n.h"
@@ -53,6 +54,7 @@ void GuiScriptingInit(ScriptInterface& scriptInterface)
 	ScriptRequest rq(scriptInterface);
 
 	JSI_GUISize::RegisterScriptClass(scriptInterface);
+	JSI_CGUISize::RegisterScriptClass(scriptInterface);
 	JSI_ConfigDB::RegisterScriptFunctions(rq);
 	JSI_Console::RegisterScriptFunctions(rq);
 	JSI_Debug::RegisterScriptFunctions(rq);
diff --git a/source/gui/SettingTypes/CGUISize.cpp b/source/gui/SettingTypes/CGUISize.cpp
index 843d200115..a914e8b7bc 100644
--- a/source/gui/SettingTypes/CGUISize.cpp
+++ b/source/gui/SettingTypes/CGUISize.cpp
@@ -1,4 +1,4 @@
-/* Copyright (C) 2021 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -142,90 +142,3 @@ bool CGUISize::FromString(const CStr& Value)
 	percent.bottom =	percents[3];
 	return true;
 }
-
-void CGUISize::ToJSVal(const ScriptRequest& rq, JS::MutableHandleValue ret) const
-{
-	const ScriptInterface& scriptInterface = rq.GetScriptInterface();
-	ret.setObjectOrNull(scriptInterface.CreateCustomObject("GUISize"));
-
-	if (!ret.isObject())
-	{
-		ScriptException::Raise(rq, "CGUISize value is not an Object");
-		return;
-	}
-
-	JS::RootedObject obj(rq.cx, &ret.toObject());
-	if (!JS_InstanceOf(rq.cx, obj, &JSI_GUISize::JSI_class, nullptr))
-	{
-		ScriptException::Raise(rq, "CGUISize value is not a CGUISize class instance");
-		return;
-	}
-
-#define P(x, y, z)\
-	if (!Script::SetProperty(rq, ret, #z, x.y)) \
-	{ \
-		ScriptException::Raise(rq, "Could not SetProperty '%s'", #z); \
-		return; \
-	}
-	P(pixel, left, left);
-	P(pixel, top, top);
-	P(pixel, right, right);
-	P(pixel, bottom, bottom);
-	P(percent, left, rleft);
-	P(percent, top, rtop);
-	P(percent, right, rright);
-	P(percent, bottom, rbottom);
-#undef P
-}
-
-bool CGUISize::FromJSVal(const ScriptRequest& rq, JS::HandleValue v)
-{
-	if (v.isString())
-	{
-		CStrW str;
-		if (!Script::FromJSVal(rq, v, str))
-		{
-			LOGERROR("CGUISize could not read JS string");
-			return false;
-		}
-
-		if (!FromString(str.ToUTF8()))
-		{
-			LOGERROR("CGUISize could not parse JS string");
-			return false;
-		}
-		return true;
-	}
-
-	if (!v.isObject())
-	{
-		LOGERROR("CGUISize value is not an String, nor Object");
-		return false;
-	}
-
-	JS::RootedObject obj(rq.cx, &v.toObject());
-	if (!JS_InstanceOf(rq.cx, obj, &JSI_GUISize::JSI_class, nullptr))
-	{
-		LOGERROR("CGUISize value is not a CGUISize class instance");
-		return false;
-	}
-
-#define P(x, y, z) \
-	if (!Script::GetProperty(rq, v, #z, x.y))\
-	{\
-		LOGERROR("CGUISize could not get object property '%s'", #z);\
-		return false;\
-	}
-
-	P(pixel, left, left);
-	P(pixel, top, top);
-	P(pixel, right, right);
-	P(pixel, bottom, bottom);
-	P(percent, left, rleft);
-	P(percent, top, rtop);
-	P(percent, right, rright);
-	P(percent, bottom, rbottom);
-#undef P
-
-	return true;
-}
diff --git a/source/gui/SettingTypes/CGUISize.h b/source/gui/SettingTypes/CGUISize.h
index d9769453d4..244d0d1e08 100644
--- a/source/gui/SettingTypes/CGUISize.h
+++ b/source/gui/SettingTypes/CGUISize.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2021 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -66,9 +66,6 @@ public:
 	{
 		return pixel == other.pixel && percent == other.percent;
 	}
-
-	void ToJSVal(const ScriptRequest& rq, JS::MutableHandleValue ret) const;
-	bool FromJSVal(const ScriptRequest& rq, JS::HandleValue v);
 };
 
 #endif // INCLUDED_CGUISIZE
diff --git a/source/gui/tests/test_GUISetting.h b/source/gui/tests/test_GUISetting.h
index 36996b0f55..e3dd5ef435 100644
--- a/source/gui/tests/test_GUISetting.h
+++ b/source/gui/tests/test_GUISetting.h
@@ -21,8 +21,11 @@
 #include "graphics/FontMetrics.h"
 #include "gui/CGUI.h"
 #include "gui/CGUISetting.h"
+#include "gui/Scripting/JSInterface_CGUISize.h"
+#include "gui/Scripting/JSInterface_GUIProxy.h"
 #include "gui/CGUIText.h"
 #include "gui/ObjectBases/IGUIObject.h"
+#include "gui/SettingTypes/CGUISize.h"
 #include "gui/SettingTypes/CGUIString.h"
 #include "i18n/L10n.h"
 #include "ps/CLogger.h"
@@ -31,7 +34,11 @@
 #include "ps/ProfileViewer.h"
 #include "ps/VideoMode.h"
 #include "renderer/Renderer.h"
+#include "scriptinterface/Object.h"
+#include "scriptinterface/ScriptContext.h"
 #include "scriptinterface/ScriptInterface.h"
+#include "scriptinterface/ScriptRequest.h"
+#include "scriptinterface/ModuleLoader.h"
 
 #include <memory>
 #include <optional>
@@ -51,11 +58,23 @@ public:
 		TestGUIObject(CGUI& gui) : IGUIObject(gui) {}
 
 		void Draw(CCanvas2D&) {}
+		void UpdateCachedSize() {
+			haveUpdateCachedSize = true;
+			IGUIObject::UpdateCachedSize();
+		}
+
+		CGUISimpleSetting<CGUISize>* GetSizeSetting() const
+		{
+			return static_cast<CGUISimpleSetting<CGUISize>*>(m_Settings.at("size"));
+		}
+
+		bool haveUpdateCachedSize{false};
 	};
 
 	void setUp()
 	{
 		g_VFS = CreateVfs();
+		TS_ASSERT_OK(g_VFS->Mount(L"", DataDir() / "mods" / "_test.gui" / "", VFS_MOUNT_MUST_EXIST));
 		TS_ASSERT_OK(g_VFS->Mount(L"", DataDir() / "mods" / "_test.minimal" / "", VFS_MOUNT_MUST_EXIST));
 		TS_ASSERT_OK(g_VFS->Mount(L"cache", DataDir() / "_testcache" / "", 0, VFS_MAX_PRIORITY));
 
@@ -106,4 +125,57 @@ public:
 		object.SetSettingFromString("A", L"ValueB", false);
 		TS_ASSERT_EQUALS(*settingB, "ValueB");
 	}
+
+	void test_setting_cguisize()
+	{
+		CGUI gui{*g_ScriptContext};
+		gui.AddObjectTypes();
+		TestGUIObject object{gui};
+
+		CGUISimpleSetting<CGUISize>* setting{object.GetSizeSetting()};
+		object.SetSettingFromString("size", L"2 2 20 20", false);
+		object.haveUpdateCachedSize = false;
+
+		ScriptRequest rq{gui.GetScriptInterface()};
+		JS::RootedValue val(rq.cx);
+		val.setObject(*object.GetJSObject());
+		JS::RootedObject global(rq.cx, rq.glob);
+		JS_DefineProperty(rq.cx, global, "testObject", val, JSPROP_ENUMERATE);
+
+		// Lazy assigment.
+		TS_ASSERT(gui.GetScriptInterface()->LoadGlobalScriptFile(L"gui/settings/cguisize/lazyassign.js"));
+		TS_ASSERT_EQUALS(setting->GetMutable().pixel, (CRect{5, 2, 20, 20}));
+		TS_ASSERT_EQUALS(setting->GetMutable().percent, (CRect{0, 0, 0, 0}));
+		TS_ASSERT_EQUALS(object.haveUpdateCachedSize, false);
+
+		// Force update of cached size.
+		object.GetComputedSize();
+		object.haveUpdateCachedSize = false;
+
+		// Compound assignment operator.
+		TS_ASSERT(gui.GetScriptInterface()->LoadGlobalScriptFile(L"gui/settings/cguisize/compoundassignmentoperator.js"));
+		TS_ASSERT_EQUALS(setting->GetMutable().pixel, (CRect{10, 2, 20, 20}));
+		TS_ASSERT_EQUALS(setting->GetMutable().percent, (CRect{0, 0, 0, 0}));
+		TS_ASSERT_EQUALS(object.haveUpdateCachedSize, false);
+
+		// Force update of cached size.
+		object.GetComputedSize();
+		object.haveUpdateCachedSize = false;
+
+		// Object assignment.
+		TS_ASSERT(gui.GetScriptInterface()->LoadGlobalScriptFile(L"gui/settings/cguisize/objectassign.js"));
+		TS_ASSERT_EQUALS(setting->GetMutable().pixel, (CRect{10, 2, 20, 20}));
+		TS_ASSERT_EQUALS(setting->GetMutable().percent, (CRect{4, 0, 0, 20}));
+		TS_ASSERT_EQUALS(object.haveUpdateCachedSize, false);
+
+		// Force update of cached size.
+		object.GetComputedSize();
+		object.haveUpdateCachedSize = false;
+
+		// assign
+		TS_ASSERT(gui.GetScriptInterface()->LoadGlobalScriptFile(L"gui/settings/cguisize/assign.js"));
+		TS_ASSERT_EQUALS(setting->GetMutable().pixel, (CRect{3, 0, 0, 2}));
+		TS_ASSERT_EQUALS(setting->GetMutable().percent, (CRect{0, 0, 0, 0}));
+		TS_ASSERT_EQUALS(object.haveUpdateCachedSize, true);
+	}
 };