Here's a little trick I used to use when buying pet store feeders. Make a circle with your fingers (like an OK sign) and figure out which fingers it takes to make the right size circle that fits that snake's empty girth. Now you have an instant reference at the pet store to compare a rat's hind end to. The other option is to grab a shoelace, wrap it around the snake's girth and make a knot. Make another knot for the other snake. Again now you have a way to actually compare.

It takes a while to be able to eyeball prey sizes. I know, I've been there and stuck with a totally incorrect size once I was home from the pet store. Darn feeder tubs should be marked "rats may be larger than they appear" LOL