diff --git a/.github/workflows/pr-size.yml b/.github/workflows/pr-size.yml
new file mode 100644
index 000000000..7a338a76e
--- /dev/null
+++ b/.github/workflows/pr-size.yml
@@ -0,0 +1,166 @@
+name: PR Size
+
+on:
+  pull_request_target:
+    types: [opened, reopened, synchronize, ready_for_review, converted_to_draft]
+
+permissions:
+  contents: read
+  issues: write
+  pull-requests: read
+
+jobs:
+  label:
+    name: Label PR size
+    runs-on: ubuntu-24.04
+    concurrency:
+      group: pr-size-${{ github.event.pull_request.number }}
+      cancel-in-progress: true
+    steps:
+      - name: Sync PR size label
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const issueNumber = context.payload.pull_request.number;
+            const additions = context.payload.pull_request.additions ?? 0;
+            const deletions = context.payload.pull_request.deletions ?? 0;
+            const changedLines = additions + deletions;
+
+            const managedLabels = [
+              {
+                name: "size:XS",
+                color: "0e8a16",
+                description: "0-9 changed lines (additions + deletions).",
+              },
+              {
+                name: "size:S",
+                color: "5ebd3e",
+                description: "10-29 changed lines (additions + deletions).",
+              },
+              {
+                name: "size:M",
+                color: "fbca04",
+                description: "30-99 changed lines (additions + deletions).",
+              },
+              {
+                name: "size:L",
+                color: "fe7d37",
+                description: "100-499 changed lines (additions + deletions).",
+              },
+              {
+                name: "size:XL",
+                color: "d93f0b",
+                description: "500-999 changed lines (additions + deletions).",
+              },
+              {
+                name: "size:XXL",
+                color: "b60205",
+                description: "1,000+ changed lines (additions + deletions).",
+              },
+            ];
+
+            const managedLabelNames = new Set(managedLabels.map((label) => label.name));
+
+            const resolveSizeLabel = (totalChangedLines) => {
+              if (totalChangedLines < 10) {
+                return "size:XS";
+              }
+
+              if (totalChangedLines < 30) {
+                return "size:S";
+              }
+
+              if (totalChangedLines < 100) {
+                return "size:M";
+              }
+
+              if (totalChangedLines < 500) {
+                return "size:L";
+              }
+
+              if (totalChangedLines < 1000) {
+                return "size:XL";
+              }
+
+              return "size:XXL";
+            };
+
+            for (const label of managedLabels) {
+              try {
+                const { data: existing } = await github.rest.issues.getLabel({
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  name: label.name,
+                });
+
+                if (
+                  existing.color !== label.color ||
+                  (existing.description ?? "") !== label.description
+                ) {
+                  await github.rest.issues.updateLabel({
+                    owner: context.repo.owner,
+                    repo: context.repo.repo,
+                    name: label.name,
+                    color: label.color,
+                    description: label.description,
+                  });
+                }
+              } catch (error) {
+                if (error.status !== 404) {
+                  throw error;
+                }
+
+                try {
+                  await github.rest.issues.createLabel({
+                    owner: context.repo.owner,
+                    repo: context.repo.repo,
+                    name: label.name,
+                    color: label.color,
+                    description: label.description,
+                  });
+                } catch (createError) {
+                  if (createError.status !== 422) {
+                    throw createError;
+                  }
+                }
+              }
+            }
+
+            const nextLabelName = resolveSizeLabel(changedLines);
+
+            const { data: currentLabels } = await github.rest.issues.listLabelsOnIssue({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: issueNumber,
+              per_page: 100,
+            });
+
+            for (const label of currentLabels) {
+              if (!managedLabelNames.has(label.name) || label.name === nextLabelName) {
+                continue;
+              }
+
+              try {
+                await github.rest.issues.removeLabel({
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  issue_number: issueNumber,
+                  name: label.name,
+                });
+              } catch (removeError) {
+                if (removeError.status !== 404) {
+                  throw removeError;
+                }
+              }
+            }
+
+            if (!currentLabels.some((label) => label.name === nextLabelName)) {
+              await github.rest.issues.addLabels({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: issueNumber,
+                labels: [nextLabelName],
+              });
+            }
+
+            core.info(`PR #${issueNumber}: ${changedLines} changed lines -> ${nextLabelName}`);
diff --git a/.github/workflows/pr-vouch.yml b/.github/workflows/pr-vouch.yml
index 976a9a097..350a71b6b 100644
--- a/.github/workflows/pr-vouch.yml
+++ b/.github/workflows/pr-vouch.yml
@@ -111,7 +111,7 @@ jobs:
               },
             ];
 
-            const managedLabelNames = managedLabels.map((label) => label.name);
+            const managedLabelNames = new Set(managedLabels.map((label) => label.name));
 
             for (const label of managedLabels) {
               try {
@@ -138,13 +138,19 @@ jobs:
                   throw error;
                 }
 
-                await github.rest.issues.createLabel({
-                  owner: context.repo.owner,
-                  repo: context.repo.repo,
-                  name: label.name,
-                  color: label.color,
-                  description: label.description,
-                });
+                try {
+                  await github.rest.issues.createLabel({
+                    owner: context.repo.owner,
+                    repo: context.repo.repo,
+                    name: label.name,
+                    color: label.color,
+                    description: label.description,
+                  });
+                } catch (createError) {
+                  if (createError.status !== 422) {
+                    throw createError;
+                  }
+                }
               }
             }
 
@@ -159,17 +165,35 @@ jobs:
               owner: context.repo.owner,
               repo: context.repo.repo,
               issue_number: issueNumber,
+              per_page: 100,
             });
 
-            const preservedLabels = currentLabels
-              .map((label) => label.name)
-              .filter((name) => !managedLabelNames.includes(name));
+            for (const label of currentLabels) {
+              if (!managedLabelNames.has(label.name) || label.name === nextLabelName) {
+                continue;
+              }
 
-            await github.rest.issues.setLabels({
-              owner: context.repo.owner,
-              repo: context.repo.repo,
-              issue_number: issueNumber,
-              labels: [...preservedLabels, nextLabelName],
-            });
+              try {
+                await github.rest.issues.removeLabel({
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  issue_number: issueNumber,
+                  name: label.name,
+                });
+              } catch (removeError) {
+                if (removeError.status !== 404) {
+                  throw removeError;
+                }
+              }
+            }
+
+            if (!currentLabels.some((label) => label.name === nextLabelName)) {
+              await github.rest.issues.addLabels({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: issueNumber,
+                labels: [nextLabelName],
+              });
+            }
 
             core.info(`PR #${issueNumber}: ${status} -> ${nextLabelName}`);
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 2021ecdcb..8b734a99b 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -8,7 +8,7 @@ You can still open an issue or PR, but please do so knowing there is a high chan
 
 If that sounds annoying, that is because it is. This project is still early and we are trying to keep scope, quality, and direction under control.
 
-PRs are automatically labeled with a `vouch:*` trust status.
+PRs are automatically labeled with a `vouch:*` trust status and a `size:*` diff size based on changed lines.
 
 If you are an external contributor, expect `vouch:unvouched` until we explicitly add you to [.github/VOUCHED.td](.github/VOUCHED.td).