diff --git a/.changeset/tricky-results-roll.md b/.changeset/tricky-results-roll.md
new file mode 100644
index 00000000000..a79f5505021
--- /dev/null
+++ b/.changeset/tricky-results-roll.md
@@ -0,0 +1,5 @@
+---
+'@shopify/app': minor
+---
+
+report file size for extensions on build and dev
diff --git a/packages/app/src/cli/services/build/bundle-size.test.ts b/packages/app/src/cli/services/build/bundle-size.test.ts
new file mode 100644
index 00000000000..af06b225854
--- /dev/null
+++ b/packages/app/src/cli/services/build/bundle-size.test.ts
@@ -0,0 +1,81 @@
+import {getBundleSize, formatBundleSize} from './bundle-size.js'
+import {describe, expect, test, vi} from 'vitest'
+import {readFile} from '@shopify/cli-kit/node/fs'
+import {deflate} from 'node:zlib'
+import {promisify} from 'node:util'
+
+const deflateAsync = promisify(deflate)
+
+vi.mock('@shopify/cli-kit/node/fs')
+
+describe('getBundleSize', () => {
+  test('returns raw and compressed sizes', async () => {
+    // Given
+    const content = 'a'.repeat(10000)
+    vi.mocked(readFile).mockResolvedValue(content as any)
+
+    // When
+    const result = await getBundleSize('/some/path.js')
+
+    // Then
+    expect(result.rawBytes).toBe(10000)
+    expect(result.compressedBytes).toBe((await deflateAsync(Buffer.from(content))).byteLength)
+    expect(result.compressedBytes).toBeLessThan(result.rawBytes)
+  })
+
+  test('compressed size uses deflate to match the backend (Ruby Zlib::Deflate.deflate)', async () => {
+    // Given
+    const content = JSON.stringify({key: 'value', nested: {array: [1, 2, 3]}})
+    vi.mocked(readFile).mockResolvedValue(content as any)
+
+    // When
+    const result = await getBundleSize('/some/path.js')
+
+    // Then
+    const expectedCompressed = (await deflateAsync(Buffer.from(content))).byteLength
+    expect(result.compressedBytes).toBe(expectedCompressed)
+  })
+})
+
+describe('formatBundleSize', () => {
+  test('returns formatted size string with raw and compressed sizes', async () => {
+    // Given
+    const content = 'x'.repeat(50000)
+    const compressedSize = (await deflateAsync(Buffer.from(content))).byteLength
+    vi.mocked(readFile).mockResolvedValue(content as any)
+
+    // When
+    const result = await formatBundleSize('/some/path.js')
+
+    // Then
+    const expectedRaw = (50000 / 1024).toFixed(1)
+    const expectedCompressed = (compressedSize / 1024).toFixed(1)
+    expect(result).toBe(` (${expectedRaw} KB original, ~${expectedCompressed} KB compressed)`)
+  })
+
+  test('formats MB for large files', async () => {
+    // Given
+    const content = 'a'.repeat(2 * 1024 * 1024)
+    const compressedSize = (await deflateAsync(Buffer.from(content))).byteLength
+    vi.mocked(readFile).mockResolvedValue(content as any)
+
+    // When
+    const result = await formatBundleSize('/some/path.js')
+
+    // Then
+    const expectedRaw = (Buffer.byteLength(content) / (1024 * 1024)).toFixed(2)
+    const expectedCompressed = (compressedSize / 1024).toFixed(1)
+    expect(result).toBe(` (${expectedRaw} MB original, ~${expectedCompressed} KB compressed)`)
+  })
+
+  test('returns empty string on error', async () => {
+    // Given
+    vi.mocked(readFile).mockRejectedValue(new Error('file not found'))
+
+    // When
+    const result = await formatBundleSize('/missing/path.js')
+
+    // Then
+    expect(result).toBe('')
+  })
+})
diff --git a/packages/app/src/cli/services/build/bundle-size.ts b/packages/app/src/cli/services/build/bundle-size.ts
new file mode 100644
index 00000000000..00edcd6a941
--- /dev/null
+++ b/packages/app/src/cli/services/build/bundle-size.ts
@@ -0,0 +1,44 @@
+import {readFile} from '@shopify/cli-kit/node/fs'
+import {outputDebug} from '@shopify/cli-kit/node/output'
+import {deflate} from 'node:zlib'
+import {promisify} from 'node:util'
+
+const deflateAsync = promisify(deflate)
+
+/**
+ * Computes the raw and compressed (deflate) size of a file.
+ * Uses the same compression algorithm as the Shopify backend (Zlib::Deflate.deflate).
+ */
+export async function getBundleSize(filePath: string) {
+  const content = await readFile(filePath)
+  const rawBytes = Buffer.byteLength(content)
+  const compressed = await deflateAsync(Buffer.from(content))
+  const compressedBytes = compressed.byteLength
+
+  return {path: filePath, rawBytes, compressedBytes}
+}
+
+/**
+ * Formats a byte count as a human-readable string (KB or MB).
+ */
+function formatSize(bytes: number) {
+  if (bytes >= 1024 * 1024) {
+    return `${(bytes / (1024 * 1024)).toFixed(2)} MB`
+  }
+  return `${(bytes / 1024).toFixed(1)} KB`
+}
+
+/**
+ * Returns a formatted bundle size suffix like " (21.4 KB original, ~8.3 KB compressed)".
+ * Returns an empty string on failure so callers can append it unconditionally.
+ */
+export async function formatBundleSize(filePath: string) {
+  try {
+    const {rawBytes, compressedBytes} = await getBundleSize(filePath)
+    return ` (${formatSize(rawBytes)} original, ~${formatSize(compressedBytes)} compressed)`
+    // eslint-disable-next-line no-catch-all/no-catch-all
+  } catch (error) {
+    outputDebug(`Failed to get bundle size for ${filePath}: ${error}`)
+    return ''
+  }
+}
diff --git a/packages/app/src/cli/services/build/extension.ts b/packages/app/src/cli/services/build/extension.ts
index d14164e17b2..97e8eca8f7a 100644
--- a/packages/app/src/cli/services/build/extension.ts
+++ b/packages/app/src/cli/services/build/extension.ts
@@ -1,3 +1,4 @@
+import {formatBundleSize} from './bundle-size.js'
 import {AppInterface} from '../../models/app/app.js'
 import {bundleExtension} from '../extensions/bundle.js'
 import {buildGraphqlTypes, buildJSFunction, runTrampoline, runWasmOpt} from '../function/build.js'
@@ -110,7 +111,8 @@ export async function buildUIExtension(extension: ExtensionInstance, options: Ex
 
   await extension.buildValidation()
 
-  options.stdout.write(`${extension.localIdentifier} successfully built`)
+  const sizeInfo = await formatBundleSize(extension.outputPath)
+  options.stdout.write(`${extension.localIdentifier} successfully built${sizeInfo}`)
 }
 
 type BuildFunctionExtensionOptions = ExtensionBuildOptions
diff --git a/packages/app/src/cli/services/dev/app-events/app-event-watcher.ts b/packages/app/src/cli/services/dev/app-events/app-event-watcher.ts
index 5a760de6332..686cfb17bdb 100644
--- a/packages/app/src/cli/services/dev/app-events/app-event-watcher.ts
+++ b/packages/app/src/cli/services/dev/app-events/app-event-watcher.ts
@@ -5,6 +5,7 @@ import {handleWatcherEvents} from './app-event-watcher-handler.js'
 import {AppLinkedInterface} from '../../../models/app/app.js'
 import {ExtensionInstance} from '../../../models/extensions/extension-instance.js'
 import {ExtensionBuildOptions} from '../../build/extension.js'
+import {formatBundleSize} from '../../build/bundle-size.js'
 import {outputDebug} from '@shopify/cli-kit/node/output'
 import {AbortSignal} from '@shopify/cli-kit/node/abort'
 import {joinPath} from '@shopify/cli-kit/node/path'
@@ -259,7 +260,8 @@ export class AppEventWatcher extends EventEmitter {
         try {
           if (this.esbuildManager.contexts?.[ext.uid]?.length) {
             await this.esbuildManager.rebuildContext(ext)
-            this.options.stdout.write(`Build successful`)
+            const sizeInfo = await formatBundleSize(ext.outputPath)
+            this.options.stdout.write(`Build successful${sizeInfo}`)
           } else {
             await this.buildExtension(ext)
           }