For young BP's the best way to judge is by weight, you want to feed your BP a rodent that weighs (or multiple rodents that together weigh) 15% of the snakes body weight.
Say you have a 100 gram BP.
100 grams x 0.15 = 15 grams
So that 100 gram BP should eat 15 grams of rodent per feeding. This doesn't work as well with adults, with adults (and it works with young BP's too) you want to feed them a rodent that is about the same width as the widest part of the BP.