It's more about the snake's weight and girth than it is about length. As for the OP, I think you are feeding your BP way too much and too big of a meal. BPs will often eat anything you put in front of them and can get very overweight and have medical problems. My biggest girl is 1600 grams and 4 years old and gets small rats weekly (and is still growing quite rapidly). She recently went to the vet and I was told she is overweight for her bone structure and that I should still cut back. If you keep stuffing your guy with medium rats at only 500 grams, he is going to end up obese.