I read somewhere last night you want it to be about 15% of the weight of your baby.

I am feeding my very small 3 year old girl BP rat pups at the moment, and they even seem big for her. Shes 30+ inches, not frail but definitely not big. Probably switching to small rats soon though)

I like using the width to measure. The prey should be between 1 to 1.5 times the width of your snake. Some people say 2 times the width but I think if its a new snake, you should take it easy until you actually get them feeding. Im no expert though!