I have a food scale that automatically shows me digital grams for weighing my BP. And i usually go by her girth to determine the size of the rat to feed Perry. As far as I know, length doesn't really matter, the size of your Bp's girth would make me think to either get a large mouse or rat pup depending on what you preferr to feed. When it get's a little older and thicker, try a fairly small adolescent rat. I'm no expert but this has always worked for me. It makes it easier since my BP is a VERY easy eater. Hope this helps!