i've had land hermit crabs for a while (although the first two I bought with a kit died within a week). I've found a ten gallon tank to be a great size for 4 or 5 hermit crabs (hermit crabs are social animals and should be kept in groups. they have been known to get lonely when not in groups which can drastically shorten their life span) you should always have at least one other shell for each hermit crab (each one bigger than the one they are in). Gravel is not very good for them. you should get a softer substrate such as sand of dirt.
happy owner of a wild type GFP axolotl named Percival and too many guppies to count.