There are a variety of small rodents which are commonly kept as pets.
The group of small mammals comprised of hamsters, gerbils, mice and rats are commonly found at pets stores, and favorites as family pets. While they are all rodents, they have varied requirements for space and social interaction, and have differing qualities as pets. The following comparison can help you decide which of these rodents might be the right pet for you.
Syrian or Golden Hamsters live on average about 2-3 years. Adults are about 6 inches in length. They are solitary animals that should be kept one to a cage. A basic minimum cage size is 12 x 18 inches (and 12 inches tall), but since hamsters are quite active, the bigger the better (and those cute modular plastic hamster cages are often too small for hamsters). Syrian hamsters that are handled from a young age usually have an agreeable temperament, but if not handled much they can be defensive and can deliver a painful bite (they are also nocturnal and can be difficult to wake or act grumpy if woken during the day). Overall hamsters are widely available and make good pets, with the advantage that they can be kept single. Also, several species of dwarf hamster are becoming more widely available. These are smaller than Syrian hamsters and are often more social, at least with others of their species. Some have a reputation as biters, although that depends a bit on the species and how much they are handled.
Gerbils live an average of 2-3 years (although up to 5 years has been reported). They have a body length of about 4 inches, with a furry tail that is almost as long as the body (never pick a gerbil up by that tail as they can shed the whole tail as a defense mechanism and it doesn't grow back!). Gerbils are very active and social so are happier as a pair or group (get all males or all females to prevent prolific breeding). Introducing adults is difficult, though, so it is best to acquire a pair (or more) that are littermates or quite young. A recommended minimum cage size for a pair or small group of gerbils is 12 by 24 inches, with a height of 12 inches, although more space would be better. With regular handling, gerbils can become quite tame, and are interesting to watch as they play and interact with their cage-mates.
Fancy Mice live 1-3 years on average, and are quite easy to keep. They are the smallest of this group with bodies that are only about 3 inches long, with a long hairless tail. They are also social and females do well in pairs or small groups (males tend to fight if kept with other males). They need a relatively small cage, minimum 12 by 18 inches (and 12 inches tall), although a larger cage is recommended if you have more than a pair. Mice can become quite tame if handled regularly but are small, fast and can be skittish so are not the best candidates for human interaction. However, they are low maintenance and quite active and playful, so they are entertaining to watch.
Rats live an average of 2-4 years, and make nice pets. They are very social, and it is best to keep a same sex pair (male or female). Since they are larger, with a body length of about 8 inches (and a hairless tail about the same), they are easy to handle. With regular handling they become very tame and enjoy human companionship, and they rarely bite. They need time outside of the cage for social interaction and exercise, and can even be taught simple tricks (they have even been compared to dogs in their ability to bond and interact with people). They do need a large cage: a tall cage with at least 24 inches by 36 inches of floor space is recommended. Due to their social nature they need more time and attention than many of the other small rodents, but make rewarding pets.