diff --git a/.github/workflows/pr-size.yml b/.github/workflows/pr-size.yml
new file mode 100644
index 0000000..1982c05
--- /dev/null
+++ b/.github/workflows/pr-size.yml
@@ -0,0 +1,103 @@
+name: PR Size Check and Label
+
+# Uses pull_request_target so it works on fork PRs (write access to labels).
+# Safe because this workflow only reads PR metadata — it never checks out untrusted code.
+on:
+  pull_request_target:
+    branches: [main]
+
+jobs:
+  label-size:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    steps:
+      - name: Calculate PR size and apply label
+        uses: actions/github-script@v8
+        with:
+          script: |
+            const prNumber = context.payload.pull_request.number;
+            const { owner, repo } = context.repo;
+
+            // Fetch file-level stats so we can exclude generated/lock files
+            const files = await github.paginate(github.rest.pulls.listFiles, {
+              owner,
+              repo,
+              pull_number: prNumber,
+              per_page: 100
+            });
+
+            const EXCLUDED = [
+              /package-lock\.json$/,
+              /yarn\.lock$/,
+              /pnpm-lock\.yaml$/,
+              /\.snap$/
+            ];
+
+            let additions = 0;
+            let deletions = 0;
+            for (const file of files) {
+              if (EXCLUDED.some(re => re.test(file.filename))) continue;
+              additions += file.additions;
+              deletions += file.deletions;
+            }
+            const totalChanges = additions + deletions;
+
+            // Remove existing size labels (catch 404 in case of race)
+            const labels = await github.rest.issues.listLabelsOnIssue({
+              owner, repo, issue_number: prNumber
+            });
+
+            for (const label of labels.data) {
+              if (label.name.startsWith('size/')) {
+                try {
+                  await github.rest.issues.removeLabel({
+                    owner, repo, issue_number: prNumber, name: label.name
+                  });
+                } catch (e) {
+                  if (e.status !== 404) throw e;
+                }
+              }
+            }
+
+            // Determine size label
+            let sizeLabel;
+            if (totalChanges <= 20) sizeLabel = 'size/xs';
+            else if (totalChanges <= 100) sizeLabel = 'size/s';
+            else if (totalChanges <= 500) sizeLabel = 'size/m';
+            else if (totalChanges <= 1000) sizeLabel = 'size/l';
+            else sizeLabel = 'size/xl';
+
+            // Ensure the label exists before applying it
+            const LABEL_COLORS = {
+              'size/xs': '3CBF00',
+              'size/s':  '5D9801',
+              'size/m':  'FBCA04',
+              'size/l':  'E67409',
+              'size/xl': 'D93F0B'
+            };
+
+            try {
+              await github.rest.issues.getLabel({ owner, repo, name: sizeLabel });
+            } catch (e) {
+              if (e.status === 404) {
+                await github.rest.issues.createLabel({
+                  owner, repo,
+                  name: sizeLabel,
+                  color: LABEL_COLORS[sizeLabel],
+                  description: `PR size: ${sizeLabel.split('/')[1].toUpperCase()}`
+                });
+              } else {
+                throw e;
+              }
+            }
+
+            await github.rest.issues.addLabels({
+              owner, repo, issue_number: prNumber, labels: [sizeLabel]
+            });
+
+            core.info(`PR has ${totalChanges} changed lines (excluding lockfiles/snapshots) → ${sizeLabel}`);
+
+            if (sizeLabel === 'size/xl') {
+              core.warning(`PR is large (${totalChanges} lines). Consider splitting into smaller PRs.`);
+            }