A gerbil is smaller than a rat but bigger than a mouse. It depends on the species. From just mice and rats the gerbil has a tail with hair and it ends with a tassel. A gerbil is around 5-6 inches long but with the tail it is around 11-12 inches. There are bigger gerbils too like the Great gerbil who has a 7-8 inches long body and 7-8 inches long tail.