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.