How big is your crab? You can use the hermit crab sizing chart here http://lennon.csufresno.edu/~esperance31/HermitCrabSizingChartV2.doc or the measurements below:

Jumbo - over 3″
Large - 3″
Med. - 2.5″
Small - 1.5″
Teeny - 1″
Micro - .5″

How much space should each of your crabs have in a tank?

Jumbo - 5 gallons each
Large - 4 gallons each
Med. - 3 gallons each
Small - 2 gallons each
Teeny/micro - 1 gallon each