The Gambian pouched rat is the largest known rat species, reaching lengths of up to 3 feet. Their average weight is between 3 and 4 pounds, although they can weigh up to 9 pounds.
Pet rats usually range in size from 9-11 inches, with their long tails adding an extra 7-9 inches. In most cases male rats tend to be larger than the females. Gambian pouched rats can be kept as pets, but the largest ones tend to live in the wild.
According to About.com, Gambian pouched rats are most commonly found in Central Africa, south of the Sahara Desert. They are often confused with African giant pouched rats, which can grow to be equally as large as Gambian rats.