ci: fix types and branches for pull request target

Author: schirrel

.github/workflows/size.yml

@@ -2,8 +2,8 @@ name: size
 on:
   # this action will error unless run in a pr context
   pull_request_target:
-    branches:
-      - main
+    types: [opened, synchronize, reopened]
+    branches: [ "main" ]
 
 env:
   BUNDLE_SIZE: true