Mice will never become too small.. That is, unless you're feeding only hoppers to adult females.. That would just be a waste of their energy.

I feed all 45+ Ball Pythons live adult mice every week and they grow wonderfully. Sometimes faster than rat eaters Juvies get 1-2 medium adult mice depending on their size. Under 250g, they get 1 adult mouse, from 300-500 they get 1-2 adults, possibly a third if they are really wanting another.If they are over 500g, but less than 1000g, they generally get 2-3. If they are adult males, they get 2 good-sized adult mice, and if they are adult females, they get 3-4 adult mice.

All in all, gram for gram, rats and mice are the same thing. I would recommend moving up to at least a medium adult mouse. Pinkies and fuzzies are for corn snakes.