To find out how large your tank is, measure the outside dimensions and:
[Use a Tank Calculator to find out]
or [Compare it to a list of common aquarium sizes]
The list is also a great way of researching sizes to find out what you can fit into your home.
How big is your crab?
You can use the hermit crab sizing chart here http://lennon.csufresno.edu/~esperance3 ... hartV2.doc or the measurements below. The measurements given are for the distance between the hermit crab's legs when they are at standing at rest.
How much space should each of your crabs have in a tank?
Please note that the above is only a guide. Every hermit crab will get along with other hermit crabs and adjust to captivity differently. Some Es and Exotics may be too crowded by this guideline, while Purple Pinchers are normally fine with being more crowded. Enclosures that are over 55 gallons should be capable of supporting more hermit crabs then this list suggests. It's up to you to pay attention to all of your individual crabs needs and wants. Remember that hermit crabs can grow quickly and you should take into consideration the amount of space they'll require over time. Please adjust your tank size and hermit crab numbers according to your situation. Aquariums that are "tall" generally support less crabs than those that are "long" or "breeder". These guidelines are minimum recommendations, and hermit crabs may do better with more space per crab and with deeper substrate depth then what is stated here.
Additional information on why crabs need so much space and why they should not be crowded can be found in JediMasterThrash's [Crabs Per Tank Guidelines] topic.