The low level Datastore API of App Engine provides a mechanism for determining the protobuf size of an entity by using the EntityTranslator class:

val protoEntity = EntityTranslator.convertToPb(entity)
if (protoEntity.serializedSize >= 1048572) {
  // ....
}

It would be good and cloud java client library to provide such API as it will let the client code determine the size of an entity before trying to persist it.