Average size of a hermit crab?

Hermit crabs vary greatly in size, depending on the species. The smallest species, such as the pea hermit crab (Pagurus bernhardus), has a shell diameter of only a few millimeters, while the largest species, such as the coconut crab (Birgus latro), can have a shell diameter of up to 12 inches. Most hermit crabs, however, are between 1 and 2 inches in length.