If you have a hard time judging how big around your snake is compared to the size of prey at the pet store, simply take the measurement around your snake at the widest part of its body. If you take a piece of string cut to that measurement to the pet store, you can do a quick check of the circumference of the prey available to find the best match. It doesn't have to be an exact measurement, however; smaller or slightly larger prey is fine too and a bit of variety never hurts.
Very small hatchlings can be started on pinky mice (newborn mice) and then graduated to larger sizes (the main terms used are fuzzies, hoppers, weaned mice or weanlings, then large and extra large adult). If you have a species of snake that grows large enough that rats will be fed eventually, it is a good idea to switch to rats while the snake is still young to get them used to eating rats rather than mice. Younger rats that are equivalent in size to older mice can be used: see FrozenFeeders.com for a sizing and equivalents chart. Rats are named with a similar progression: pinkies, fuzzies, pups, weanlings, then small through extra large adults).
Snakes have unique jaws that allow them to swallow prey that is larger than their head, but your snake may have difficulties digesting overly large items, resulting in regurgitation.