diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 38f68f4..a18087b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -119,7 +119,7 @@ jobs:
               echo "Largest JS and CSS files:"
               echo
               echo '```'
-              find dist -type f \( -name '*.js' -o -name '*.css' \) -printf '%s %p\n' | sort -rn | head -15 | awk '{printf "%8.1f KB  %s\n", $1/1024, $2}'
+              find dist -type f \( -name '*.js' -o -name '*.css' \) -printf '%s %p\n' | sort -rn | awk 'NR<=15 {printf "%8.1f KB  %s\n", $1/1024, $2}'
               echo '```'
             else
               echo
diff --git a/eslint.config.js b/eslint.config.js
index b0f135f..56bb89e 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -34,6 +34,11 @@ export default tseslint.config(
     },
   },
 
+  {
+    files: ['scripts/**/*.mjs'],
+    languageOptions: { globals: globals.node },
+  },
+
   {
     files: ['**/*.{jsx,tsx}'],
     ...react.configs.flat.recommended,
diff --git a/package.json b/package.json
index 80e8dcd..48cf329 100644
--- a/package.json
+++ b/package.json
@@ -19,7 +19,9 @@
     "lint": "eslint .",
     "format:check": "prettier --check .",
     "knip": "knip",
-    "prepare": "node scripts/install-hooks.mjs"
+    "prepare": "node scripts/install-hooks.mjs",
+    "size": "pnpm build && node scripts/check-bundle.mjs",
+    "size:update": "pnpm build && node scripts/check-bundle.mjs --update"
   },
   "dependencies": {
     "@salla.sa/twilight-components-react": "^3.0.2",
diff --git a/scripts/check-bundle.mjs b/scripts/check-bundle.mjs
new file mode 100644
index 0000000..d72ad4a
--- /dev/null
+++ b/scripts/check-bundle.mjs
@@ -0,0 +1,121 @@
+// Compares the client build output (dist/client/assets) with size-budget.json.
+// Usage: node scripts/check-bundle.mjs [--update]   (--update rewrites the budget: current size + margin)
+import { readFileSync, readdirSync, writeFileSync, mkdirSync } from 'node:fs';
+import { join, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { gzipSync, brotliCompressSync } from 'node:zlib';
+
+const root = process.env.BUNDLE_ROOT ?? join(dirname(fileURLToPath(import.meta.url)), '..');
+const assetsDir = join(root, 'dist/client/assets');
+const budgetPath = join(root, 'size-budget.json');
+const reportPath = join(root, 'dist/size-report.json');
+const MARGIN = 0.05;
+const TRACK_MIN_GZIP = 4096;
+
+const kb = (n) => (n / 1024).toFixed(1) + ' kB';
+const diff = (n) => (n > 0 ? '+' : n < 0 ? '-' : '') + kb(Math.abs(n));
+
+function measure() {
+  let files;
+  try {
+    files = readdirSync(assetsDir).filter((f) => /\.(js|css)$/.test(f));
+  } catch {
+    throw new Error(`No client build at ${assetsDir}. Run "pnpm build" first.`);
+  }
+  if (!files.some((f) => f.endsWith('.js'))) {
+    throw new Error(`No JS assets in ${assetsDir}; the build layout may have changed.`);
+  }
+  const chunks = {};
+  const totals = { js: { gzip: 0, brotli: 0 }, css: { gzip: 0, brotli: 0 } };
+  for (const file of files) {
+    const buf = readFileSync(join(assetsDir, file));
+    const gzip = gzipSync(buf, { level: 9 }).length;
+    const brotli = brotliCompressSync(buf).length;
+    const kind = file.endsWith('.css') ? 'css' : 'js';
+    totals[kind].gzip += gzip;
+    totals[kind].brotli += brotli;
+    // Drop every embedded content hash so a chunk keeps one budget across builds.
+    const name = file.replace(/(-[A-Za-z0-9_-]{8})+(\.(?:js|css))$/, '$2');
+    const prev = chunks[name] ?? { gzip: 0, brotli: 0 };
+    chunks[name] = { gzip: prev.gzip + gzip, brotli: prev.brotli + brotli };
+  }
+  return { chunks, totals };
+}
+
+const { chunks, totals } = measure();
+const withMargin = (s) => ({
+  gzip: Math.ceil(s.gzip * (1 + MARGIN)),
+  brotli: Math.ceil(s.brotli * (1 + MARGIN)),
+});
+
+if (process.argv.includes('--update')) {
+  const tracked = Object.fromEntries(
+    Object.entries(chunks)
+      .filter(([, s]) => s.gzip >= TRACK_MIN_GZIP)
+      .sort(([a], [b]) => a.localeCompare(b))
+      .map(([n, s]) => [n, withMargin(s)])
+  );
+  const budget = {
+    margin: MARGIN,
+    untrackedChunk: { gzip: TRACK_MIN_GZIP * 2, brotli: TRACK_MIN_GZIP * 2 },
+    totals: { js: withMargin(totals.js), css: withMargin(totals.css) },
+    chunks: tracked,
+  };
+  writeFileSync(budgetPath, JSON.stringify(budget, null, 2) + '\n');
+  console.log(`Wrote ${budgetPath} (current size + ${MARGIN * 100}% margin)`);
+  process.exit(0);
+}
+
+const budget = JSON.parse(readFileSync(budgetPath, 'utf8'));
+const rows = [];
+const add = (name, cur, bud) => {
+  for (const metric of ['gzip', 'brotli']) {
+    rows.push({
+      name,
+      metric,
+      current: cur[metric],
+      budget: bud[metric],
+      diff: cur[metric] - bud[metric],
+      ok: cur[metric] <= bud[metric],
+    });
+  }
+};
+add('TOTAL js', totals.js, budget.totals.js);
+add('TOTAL css', totals.css, budget.totals.css);
+for (const [name, cur] of Object.entries(chunks)) {
+  const bud = budget.chunks[name] ?? budget.untrackedChunk;
+  // Small chunks without their own budget only matter if they grow past the untracked limit.
+  add(budget.chunks[name] ? name : `${name} (new)`, cur, bud);
+}
+for (const name of Object.keys(budget.chunks)) {
+  if (!chunks[name])
+    console.warn(`Budgeted chunk not found in build (renamed or removed?): ${name}`);
+}
+
+const shown = rows.filter(
+  (r) => r.name.startsWith('TOTAL') || budget.chunks[r.name.replace(' (new)', '')] || !r.ok
+);
+const width = Math.max(...shown.map((r) => r.name.length));
+console.log(
+  `${'chunk'.padEnd(width)}  type    ${'current'.padStart(10)} ${'budget'.padStart(10)} ${'diff'.padStart(10)}  status`
+);
+for (const r of shown) {
+  console.log(
+    `${r.name.padEnd(width)}  ${r.metric.padEnd(6)}  ${kb(r.current).padStart(10)} ${kb(r.budget).padStart(10)} ${diff(r.diff).padStart(10)}  ${r.ok ? 'ok' : 'OVER'}`
+  );
+}
+
+const failures = rows.filter((r) => !r.ok);
+mkdirSync(dirname(reportPath), { recursive: true });
+writeFileSync(
+  reportPath,
+  JSON.stringify({ passed: failures.length === 0, totals, chunks, rows, failures }, null, 2) + '\n'
+);
+console.log(`\nReport: ${reportPath}`);
+if (failures.length) {
+  console.error(
+    `\nBundle budget exceeded in ${failures.length} measurement(s). If the growth is intended, run "pnpm size:update" and commit size-budget.json.`
+  );
+  process.exit(1);
+}
+console.log('Bundle within budget.');
diff --git a/size-budget.json b/size-budget.json
new file mode 100644
index 0000000..311c613
--- /dev/null
+++ b/size-budget.json
@@ -0,0 +1,67 @@
+{
+  "margin": 0.05,
+  "untrackedChunk": {
+    "gzip": 8192,
+    "brotli": 8192
+  },
+  "totals": {
+    "js": {
+      "gzip": 449837,
+      "brotli": 390723
+    },
+    "css": {
+      "gzip": 73485,
+      "brotli": 54761
+    }
+  },
+  "chunks": {
+    "app.css": {
+      "gzip": 73485,
+      "brotli": 54761
+    },
+    "chunk.js": {
+      "gzip": 19663,
+      "brotli": 17321
+    },
+    "fslightbox.js": {
+      "gzip": 10532,
+      "brotli": 9332
+    },
+    "hydrate.js": {
+      "gzip": 44368,
+      "brotli": 38659
+    },
+    "index.client.js": {
+      "gzip": 30733,
+      "brotli": 27528
+    },
+    "OrderSinglePage.js": {
+      "gzip": 6113,
+      "brotli": 5460
+    },
+    "product-listing.js": {
+      "gzip": 5231,
+      "brotli": 4661
+    },
+    "shared-home.js": {
+      "gzip": 151357,
+      "brotli": 129188
+    },
+    "shared-product.js": {
+      "gzip": 8778,
+      "brotli": 7808
+    },
+    "ThankYouPage.js": {
+      "gzip": 6215,
+      "brotli": 5178
+    },
+    "vendor-react.js": {
+      "gzip": 58600,
+      "brotli": 50727
+    },
+    "vendor-router.js": {
+      "gzip": 30816,
+      "brotli": 27888
+    }
+  }
+}
diff --git a/tests/scripts/check-bundle.test.ts b/tests/scripts/check-bundle.test.ts
new file mode 100644
index 0000000..166c477
--- /dev/null
+++ b/tests/scripts/check-bundle.test.ts
@@ -0,0 +1,98 @@
+// @vitest-environment node
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { spawnSync } from 'node:child_process';
+import { randomBytes } from 'node:crypto';
+import { mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join, resolve } from 'node:path';
+
+const script = resolve(__dirname, '../../scripts/check-bundle.mjs');
+let root: string;
+
+const asset = (name: string, bytes: number) =>
+  writeFileSync(join(root, 'dist/client/assets', name), randomBytes(bytes));
+const budget = (value: unknown) =>
+  writeFileSync(join(root, 'size-budget.json'), JSON.stringify(value));
+const run = (...args: string[]) =>
+  spawnSync('node', [script, ...args], {
+    env: { ...process.env, BUNDLE_ROOT: root },
+    encoding: 'utf8',
+  });
+const limits = (gzip: number) => ({ gzip, brotli: gzip });
+const baseBudget = (chunk: number, total = 100000) => ({
+  untrackedChunk: limits(8192),
+  totals: { js: limits(total), css: limits(total) },
+  chunks: { 'vendor.js': limits(chunk) },
+});
+
+beforeEach(() => {
+  root = mkdtempSync(join(tmpdir(), 'bundle-'));
+  mkdirSync(join(root, 'dist/client/assets'), { recursive: true });
+});
+afterEach(() => rmSync(root, { recursive: true, force: true }));
+
+describe('scripts.size.check-bundle', () => {
+  it('passes when every chunk and total is under budget, and writes the report', () => {
+    asset('vendor-AAAAAAAA.js', 5000);
+    budget(baseBudget(6000));
+    const res = run();
+    expect(res.status).toBe(0);
+    const report = JSON.parse(readFileSync(join(root, 'dist/size-report.json'), 'utf8'));
+    expect(report.passed).toBe(true);
+    expect(report.failures).toEqual([]);
+  });
+
+  it('fails when a chunk is above its budget', () => {
+    asset('vendor-AAAAAAAA.js', 5000);
+    budget(baseBudget(4000));
+    const res = run();
+    expect(res.status).toBe(1);
+    const report = JSON.parse(readFileSync(join(root, 'dist/size-report.json'), 'utf8'));
+    expect(report.failures.map((f: { name: string }) => f.name)).toContain('vendor.js');
+  });
+
+  it('fails when the JS total is above its budget', () => {
+    asset('vendor-AAAAAAAA.js', 5000);
+    budget(baseBudget(6000, 3000));
+    expect(run().status).toBe(1);
+  });
+
+  it('matches a chunk across rebuilds by dropping every embedded hash', () => {
+    asset('vendor-AAAAAAAA-BBBBBBBB.js', 5000);
+    budget(baseBudget(6000));
+    expect(run().status).toBe(0);
+    rmSync(join(root, 'dist/client/assets/vendor-AAAAAAAA-BBBBBBBB.js'));
+    asset('vendor-CCCCCCCC-D_E-F0Gh.js', 7000);
+    expect(run().status).toBe(1);
+  });
+
+  it('holds a new chunk without its own budget to the untracked limit', () => {
+    asset('vendor-AAAAAAAA.js', 5000);
+    asset('extra-AAAAAAAA.js', 9000);
+    budget(baseBudget(6000));
+    const res = run();
+    expect(res.status).toBe(1);
+    expect(res.stdout).toContain('extra.js (new)');
+  });
+
+  it('fails instead of passing when the build has no JS assets', () => {
+    budget(baseBudget(6000));
+    expect(run().status).toBe(1);
+  });
+
+  it('--update writes the current size plus the margin, keeping only chunks of 4 kB gzip or more', () => {
+    asset('vendor-AAAAAAAA.js', 5000);
+    asset('tiny-AAAAAAAA.js', 500);
+    expect(run('--update').status).toBe(0);
+    const written = JSON.parse(readFileSync(join(root, 'size-budget.json'), 'utf8'));
+    expect(Object.keys(written.chunks)).toEqual(['vendor.js']);
+    expect(written.chunks['vendor.js'].gzip).toBeGreaterThan(5000);
+    expect(written.chunks['vendor.js'].gzip).toBeLessThan(5000 * 1.06 + 100);
+    expect(run().status).toBe(0);
+  });
+
+  it('--update refuses to write a budget from an empty build', () => {
+    expect(run('--update').status).toBe(1);
+    expect(() => readFileSync(join(root, 'size-budget.json'))).toThrow();
+  });
+});