You can go with the diameter rule. Don't feed him a prey item that is larger than the diameter of his body.
If you want specific sizes, I go with rat pups up to 350 grams, rat weanlings up to 700 grams, rat smalls for the rest of their life if it's a male. If it is a female I will go up to rat mediums when they reach about 1200 grams and then for the remainder of their life.