since bp's are very timid, shy creatures they wont eat something if they think it is too big. these snakes scare easy.
the rule of thumb is offer a prey item about the size of, or a little smaller then the thickest part of the snake's body.
my girl is REALLY anal about size and although she CAN take something appropriately sized she wont. she'll only accept the smaller one's. when i buy feeders i have to ask for the smallest of their smalls.
i have recently, as of 2 feedings ago, been able to bump up the size of the rats i buy cuz the smallest one's werent leaving any kind of a bulge in my girl's belly.