General rule of thumb is the girth of the sanke at its largest point for prey items. Just remember that the legs do not count since they will be laid flat next to the snake after constricting. I personally believe in feeding rats from day one. Rats are higher in protien and lower in rat the mice. So your snakes grow faster with the same amount of food. You can start with pups and go up from there. Good luck with switching to rats it seems to be the time that they will go off feed. I would have some chicken broth handy just encase. The smell will sometimes work to get a feeding responce.