Well its kind of tricky because if your snake is overweight already, then judging the size of the food by either its girth or weight will give you false readings and you will keep overfeeding it and just increase its girth and weight and keep feeding larger and larger if that makes sense. My advice personally would start with a small rat and then wait till he takes a poop or goes into hunting mode and starts looking around for food. Then you can adjust based on that.