I've always followed the rule of food being no larger than the largest part of the body. I consider this a fact for the colubrids and most snakes, but I find that with the Ball Python, I go by the size of the head. I feed no larger than twice the size of the snake's head. Some females can be very broad and I'm not sure they should eat prey that is as large as their body. I'm not saying I'm right, it's just that's what I do.
One man's opinion,
Jim Smith