diff --git a/CHANGELOG.md b/CHANGELOG.md
index c85d93118..b70f44f42 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -64,6 +64,7 @@
 - Honor `.pen` frame layout defaults and sizing and padding shorthands so imported auto-layout frames keep their computed dimensions and child positions. (#564)
 - Avoid macOS Keychain prompts during credential status checks and pause repeated credential access after failures until explicitly retried from Settings.
 
+- Honor explicit Design JSX instance dimensions and preserve authored overrides through component synchronization.
 - Route browser Command/Ctrl plus and minus shortcuts to canvas zoom instead of page zoom.
 - Resolve `$name` references in imported `.pen` fills, stroke fills, font families, dimensions, and spacing without requiring a `--` prefix. (#563)
 - Resolve bound fields in each layer’s mode, keep variable edits scoped and undoable, and make broken bindings visible and recoverable.
diff --git a/packages/core/src/design-jsx/renderer.ts b/packages/core/src/design-jsx/renderer.ts
index 927e51365..95bc566e0 100644
--- a/packages/core/src/design-jsx/renderer.ts
+++ b/packages/core/src/design-jsx/renderer.ts
@@ -430,13 +430,34 @@ async function renderInstanceNode(
     const label = typeof ref === 'string' || typeof ref === 'number' ? String(ref) : ''
     throw new Error(`<Instance> component not found: ${label}`)
   }
-  const overrides = {
+  const overrides: Partial<SceneNode> = {
     ...propsToOverrides(props, false, parentLayout),
     ...componentMetadata(props, 'INSTANCE', componentPropertyScope(graph, parentId))
   }
+  // Instances inherit their container layout, but explicitly authored dimensions
+  // must also replace the inherited sizing mode on that axis.
+  const layout = overrides.layoutMode ?? component.layoutMode
+  if (layout !== 'NONE') {
+    const axes =
+      layout === 'HORIZONTAL'
+        ? (['primaryAxisSizing', 'counterAxisSizing'] as const)
+        : (['counterAxisSizing', 'primaryAxisSizing'] as const)
+    for (const [dimension, field] of [
+      ['w', axes[0]],
+      ['h', axes[1]]
+    ] as const) {
+      const value = props[dimension]
+      if (typeof value === 'number') overrides[field] = 'FIXED'
+      else if (value === 'hug' || value === 'fill') overrides[field] = 'HUG'
+    }
+  }
   const instance =
     graph.createInstance(component.id, parentId, overrides) ?? graph.createNode('FRAME', parentId)
   try {
+    for (const [field, value] of Object.entries(overrides)) {
+      setInstanceOverride(instance.instanceOverrides, instance.id, instance.id, field, value)
+    }
+    graph.updateNode(instance.id, { instanceOverrides: instance.instanceOverrides })
     applyBindings(graph, instance.id, bindings)
     applyInstanceOverrides(graph, instance, tree.props.overrides)
     assignComponentProperties(graph, instance, props.properties)
diff --git a/tests/e2e/canvas/instance-sizing.spec.ts b/tests/e2e/canvas/instance-sizing.spec.ts
new file mode 100644
index 000000000..b7780762d
--- /dev/null
+++ b/tests/e2e/canvas/instance-sizing.spec.ts
@@ -0,0 +1,26 @@
+import { expect, test, useEditorSetupWithClear } from '#tests/e2e/fixtures'
+import type * as SizingFixture from '#tests/helpers/canvas/instance-sizing'
+
+test.use({ viewport: { width: 900, height: 700 } })
+
+const editor = useEditorSetupWithClear('/?test&no-chrome&no-rulers')
+
+test('Fill instances wrap inside narrower parents after component synchronization', async () => {
+  await editor.page.evaluate(async () => {
+    const store = window.openPencil?.getStore?.()
+    if (!store) throw new Error('OpenPencil store not initialized')
+    const fixtureURL = '/tests/helpers/canvas/instance-sizing.ts'
+    const { createInstanceSizingScene }: typeof SizingFixture = await import(fixtureURL)
+    const { boardId, componentId } = await createInstanceSizingScene(
+      store.graph,
+      store.state.currentPageId
+    )
+    await store.loadFontsForNodes([boardId, componentId])
+    store.clearSelection()
+    store.zoomToFit()
+    store.requestRender()
+  })
+  await editor.canvas.waitForRender()
+  editor.canvas.assertNoErrors()
+  expect(await editor.canvas.screenshotCanvasRegion()).toMatchSnapshot('instance-fill-sizing.png')
+})
diff --git a/tests/e2e/canvas/instance-sizing.spec.ts-snapshots/instance-fill-sizing-openpencil-darwin.png b/tests/e2e/canvas/instance-sizing.spec.ts-snapshots/instance-fill-sizing-openpencil-darwin.png
new file mode 100644
index 000000000..7ad08de0e
Binary files /dev/null and b/tests/e2e/canvas/instance-sizing.spec.ts-snapshots/instance-fill-sizing-openpencil-darwin.png differ
diff --git a/tests/engine/render/jsx/instance-sizing.test.ts b/tests/engine/render/jsx/instance-sizing.test.ts
new file mode 100644
index 000000000..472e8f43c
--- /dev/null
+++ b/tests/engine/render/jsx/instance-sizing.test.ts
@@ -0,0 +1,49 @@
+import { expect, test } from 'bun:test'
+
+import { Component, Frame, Instance, Rectangle, renderTree } from '@open-pencil/core/design-jsx'
+import { computeAllLayouts } from '@open-pencil/core/layout'
+
+import { getNodeOrThrow } from '#tests/helpers/assert'
+import { makeSceneGraph } from '#tests/helpers/scene'
+
+for (const flex of ['row', 'col'] as const) {
+  test(`instances fill their parent across inherited fixed ${flex} dimensions`, async () => {
+    const graph = makeSceneGraph()
+    const component = await renderTree(
+      graph,
+      Component({ flex, w: 280, h: 100, children: Rectangle({ w: 20, h: 20 }) })
+    )
+    const parent = await renderTree(
+      graph,
+      Frame({
+        flex: 'col',
+        w: 220,
+        h: 180,
+        children: Instance({ of: component.id, w: 'fill', h: 'fill' })
+      })
+    )
+    const instance = graph.getChildren(parent.id)[0]
+    expect(instance.width).toBe(220)
+    expect(instance.height).toBe(180)
+    graph.syncInstances(component.id)
+    computeAllLayouts(graph)
+    expect(instance.width).toBe(220)
+    expect(instance.height).toBe(180)
+  })
+
+  test(`explicit instance dimensions override inherited Hug ${flex} sizing`, async () => {
+    const graph = makeSceneGraph()
+    const component = await renderTree(
+      graph,
+      Component({ flex, w: 'hug', h: 'hug', children: Rectangle({ w: 20, h: 20 }) })
+    )
+    const result = await renderTree(graph, Instance({ of: component.id, w: 120, h: 80 }))
+    const instance = getNodeOrThrow(graph, result.id)
+    expect(instance.width).toBe(120)
+    expect(instance.height).toBe(80)
+    graph.syncInstances(component.id)
+    computeAllLayouts(graph)
+    expect(instance.width).toBe(120)
+    expect(instance.height).toBe(80)
+  })
+}
diff --git a/tests/helpers/canvas/instance-sizing.ts b/tests/helpers/canvas/instance-sizing.ts
new file mode 100644
index 000000000..60801e93c
--- /dev/null
+++ b/tests/helpers/canvas/instance-sizing.ts
@@ -0,0 +1,65 @@
+import { Component, Frame, Instance, Text, renderTree } from '@open-pencil/core/design-jsx'
+import type { SceneGraph } from '@open-pencil/scene-graph'
+
+export async function createInstanceSizingScene(graph: SceneGraph, parentId: string) {
+  const library = graph.addPage('Sizing component')
+  const component = await renderTree(
+    graph,
+    Component({
+      name: 'Note',
+      flex: 'col',
+      w: 280,
+      h: 'hug',
+      p: 16,
+      bg: '#FFFFFF',
+      rounded: 10,
+      properties: [{ id: 'message', name: 'Message', type: 'TEXT', defaultValue: 'A short note.' }],
+      children: Text({
+        name: 'Message',
+        w: 'fill',
+        font: 'Inter',
+        size: 14,
+        lineHeight: 20,
+        color: '#252A31',
+        propertyRefs: [{ propertyId: 'message', field: 'TEXT' }],
+        children: 'A short note.'
+      })
+    }),
+    { parentId: library.id }
+  )
+  const board = await renderTree(
+    graph,
+    Frame({
+      name: 'Instance sizing',
+      flex: 'row',
+      w: 'hug',
+      h: 'hug',
+      gap: 32,
+      p: 32,
+      bg: '#E9E7E2',
+      children: [280, 220].map((width) =>
+        Frame({
+          name: `Container ${width}`,
+          flex: 'col',
+          w: width,
+          h: 'hug',
+          gap: 12,
+          children: [
+            Text({ children: `${width}px container`, font: 'Inter', size: 12, color: '#626975' }),
+            Instance({
+              of: component.id,
+              w: 'fill',
+              properties: {
+                message:
+                  'The blue feels right. Give the date a little more room at the bottom, and keep the quieter version for the print edition.'
+              }
+            })
+          ]
+        })
+      )
+    }),
+    { parentId }
+  )
+  graph.syncInstances(component.id)
+  return { boardId: board.id, componentId: component.id }
+}