I'd use a 10 or a 20 if I had one laying about. You can either just mound sand in one end or put it on an incline w/ sand in the higher end and not enough water to cover it. Go small for ease of setup, but plan on pretty frequent water changes - because it's small...
I have no experience in raising crab larvae, but it's what I'd try unless offered a better game plan by someone who actually *knows* what they're talking about.