Full disclosure: I am a first-time owner and I adopted my snake as an adult, so I have no hands-on experience with handling/feeding baby BPs. That said, the general rule of thumb that I've heard is that you want to feed something roughly as big around as the widest part of the snake's body, or just a hair larger. I'm sure some of the more knowledgeable forum members can weigh in on this, but that should give you something to aim for at least. Hope that helps!