My BP was born in September 2003 and is currently about 22 inches long. The fact that it's a he would also be part of it, as males are smaller. Mine is currently eating the 2nd from largest size of adult mice right now, which are approximately the same girth as the widest part of his body. I am going to keep mine on mice as long as possible, as in case I have difficulty switching him over, I'll want to use the 2 prey item switcharoo method on him. I would want the rat I use in the switcharoo to be fully weaned with fully developed bones, so it'll be a while before his ideal prey size is twice that. I have yet to see any formal scientific data supporting the nutritional superiority of either mice or rats, so I do not feel any urgency to make the switch.