The guide line of the thickness part of the snakes body is accurate. Thats what most people do for judging prey size.
I would feed her a mouse crawler or hopper or a 15-20 day old rat pup.
I like feeding rats right away because when that BP gets big its very annoying to fed so many adult mice for an adult BP to benefit from.
Weaning BPs from mice to rats is sometimes hard.
Are you feeding live or frozen thawed, and how often will you be feeding?