diff --git a/.github/workflows/size.yml b/.github/workflows/size.yml
index d31fce4e4..090efd8ee 100644
--- a/.github/workflows/size.yml
+++ b/.github/workflows/size.yml
@@ -53,9 +53,14 @@ jobs:
         egress-policy: audit # TODO: change to 'egress-policy: block' after couple of runs
 
     - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+    - name: Checkout Pull Request
+      if: ${{ github.event_name == 'pull_request_target' }}
+      run: |
+        git fetch origin pull/${{ github.event.pull_request.number }}/head
+        git checkout FETCH_HEAD
     - name: Run
       env:
-        GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+        PR_BODY: "${{ github.event.pull_request.body }}"
         PR_NUMBER: "${{ github.event.pull_request.number }}"
       run: |
         ./script/check-size
diff --git a/script/check-size b/script/check-size
index 149c69930..95c1955c5 100755
--- a/script/check-size
+++ b/script/check-size
@@ -32,7 +32,7 @@ set -euo pipefail
 OT_TMP_DIR=/tmp/ot-size-report
 readonly OT_TMP_DIR
 
-OT_SHA_NEW=${GITHUB_SHA:-$(git rev-parse HEAD)}
+OT_SHA_NEW="$(git rev-parse HEAD)"
 readonly OT_SHA_NEW
 
 OT_SHA_OLD="${OT_SHA_OLD:-$(git cat-file -p "${OT_SHA_NEW}" | grep 'parent ' | head -n1 | cut -d' ' -f2)}"
diff --git a/script/git-tool b/script/git-tool
index f0f057ee8..560e8f3c3 100755
--- a/script/git-tool
+++ b/script/git-tool
@@ -54,6 +54,14 @@ get_pr_body()
 {
     local pr
 
+    if [[ -z ${PR_BODY+x} ]]; then
+        echo 'PR_BODY is not set' >&2
+    else
+        echo 'PR_BODY is set' >&2
+        echo "${PR_BODY}"
+        return 0
+    fi
+
     [[ ${GITHUB_ACTIONS+x} && ${GITHUB_REF-} =~ ^refs/pull/[0-9]+/merge ]] || {
         echo 'Not a pull request.' >&2
         return 0