diff --git a/service/resources/openapi_v3.yml b/service/resources/openapi_v3.yml
index 95b1547..71f3267 100644
--- a/service/resources/openapi_v3.yml
+++ b/service/resources/openapi_v3.yml
@@ -770,6 +770,7 @@ components:
           type: string
         size:
           type: integer
+          format: int64
           description: size in bytes
     # --- GlobusTransferItem -------------------------------------------------------------
     GlobusTransferItem: