I usually put my snakes on weaned rats at around 250grams, and step them up to small rats at 400grams. Anything over 600grams gets a medium rat, and all of my larger 900gram+ snakes eat large rats.

It usually takes my snakes about 4-5 days to process that, and then about day 7-8 they are ready to eat again.

The only advice I can give you on this subject really is to maintain a weight, feed, deficate and shed schedule so that you can keep track of how quickly your BP is growing or if it isn't growing, and then just base you food choice on that.

Oh and one other thing is pay attention to how quickly they swallow the prey after they kill it. Because if they are struggling for 15 minutes to swallow something, then it's prolly to big, and you should step down a size. My snakes put down their dinner in about 5 minutes on avg. I've had snakes that have taken 15 minutes to put down a meal, and then it took them almost 2 weeks to deficate, so I had to step them down a size.