Using the gauge of the snakes largest girth is a visual aid to help a person visualize an appropriately size meal for a healthy snake. If the snake is very thin/underweight, using this gauge could cause you to underfeed him. He has the head and neck of a much thicker, heavier snake, so he is capable of swallowing larger prey. However, he may be weak or emaciated if he is very thin, so start off by feeding more frequently or multiple small prey, then try to feed him something a little bigger once his strength is up. The previous post about small rats to start, then med rats sounds about right.