HCA: Hermit Crab Association

Hi guys, My daughter talked us into getting 1 hermit crab for X-mas for her :D I need all the help I can get. I've read all I can and I think i know more or less what we need for Santa to bring, but for 1 hermit crab do I need 10 gallon tank or 5 gallon would be enough? thanks a lot feel free to e-m...