well the general rule is the food should ne no bigger that 1-1.5 times bigger than the snake. Once mine get to 3 pinkies in one feeding I will switch them to fuzzies. Another thing to consider is how long it takes the snake to digest the food. If the buldge is gone after one day your snake will need more. That is just what i do, I am sure you'll get a million different responces to this!!
HTH
Ken