Most snakes should be fed a size of prey that is about the same size around as snake's body at its widest point. This means that as a snake grows, you will have to adjust the size of prey fed to the snake to accommodate the snake's increasing size. Choosing prey that's too large can result in serious health issues for your snake. Injuries are more likely if the prey is too large and they can cause gut impaction.
Choosing the Prey Size
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. When in doubt, ask the pet store clerk for help.
Very small hatchlings can be started on pinky mice (newborn mice) and then graduate to larger sizes. The main terms used for feeder mice are fuzzies, hoppers, weaned mice or weanlings, then large and extra large adult.
Feeding Rats to Snakes
If you have a species of snake that grows large enough that they will eventually eat rats, it is a good idea to switch to rats while the snake is still young in order to get them used to eating rats rather than mice. While mice are very timid creatures with few defensive capabilities, rats are very inquisitive and a larger rat can do serious damage to your snake. You want to get your snake used to eating prey that can fight back while they're still young. Younger rats that are equivalent in size to older mice, so when your snake has graduated to larger mice, it's a good time to make the rodent switch. Feeder rats are named with a similar progression as mice: pinkies, fuzzies, pups, weanlings, then small through extra large adults.
How to Prevent Snake Regurgitation
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. Another common cause of regurgitation is handling your snake too soon after it's eaten. While every snake is different, you should wait at least 48 hours after feeding to handle your pet. If he regurgitates when you handle him after 48 hours try giving him another day of rest.
Snakes will also regurgitate due to stress. If your snake is stressed by loud noises or people tapping on the glass of their tank, try putting them in a quiet area after feeding and giving them space. Your snake will be ready to play in a couple days. Regurgitation takes a lot out of snakes, so wait about two weeks before trying to feed them again. When you do feed them, make the meal smaller.
It's important to understand the difference between regurgitation and vomiting in your snake. Regurgitation happens before the food reaches the stomach. When a snake expells digested food that's been in its stomach it's called vomiting and is usually a sign of illness. Vomiting is a bigger issue than regurgitation because the snake loses a lot of amino acids that it needs to survive in the process. If your snake is vomiting or repeatedly regurgitating it's food, get it to a vet immediately. Vomiting and regurgitation can both be fatal to snakes.