A good way to judge mass is to compare the base of the main beam to the eye and nose. For instance, if the base of the beam is about the diameter of the eye then it sould be around 4" in diameter. if the base is about the same diameter as the distance between the nostrils it should be around 5" in diameter.
I have a really good slide show about scoring on the hoof. I'll send it when I get home.