Why don't you just feed her pray that is the same girth as she is around the thickest part of her body? It's a lot easier to pick out rats that way.

Also, you might want to stick to rats if that is what she eats. Once you get her on mice, it might be hard to switch her back to rats, and mice will become a pain in the behind in the long-run. You'll have to buy more mice at a time to feed her because they will not be big enough. If mice is more convenient for you to find, then go for it, but keep in mind that you'll be feeding her a bunch of mice when you could be feeding just one appropriately sized rat.