An adult hamster will reach about 3 inches in length. Full grown they are much smaller than rats or guinea pigs and are very entertaining with minimal up-keep so are great pets for children.
Black bear hamsters can reach up 5 inches. Another way to size it on average is the size of your hand. I myself have and adult black bear hamster named Zack, and he is about the size of my palm. (About 3 1/2 inches)
