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.
According to Smithsonian Science, the Bosavi wooly rat was discovered living in one section of the New Guinea rain forest in 2009. It grows to 2.5 pounds in weight and at least 32 inches in length. According to members of the team that found the giant rats, Bosavi wooly rats are tame and show no fear of humans.