I've had a crocea clam in my 20 gallon nano for over a year now, and he seems to be doing just fine. The tank has 120watts PC light, and he's up on the rock near the top. He's not growing very much, but has better color than when I first got him.
I would reccommend 20 gallons minimum temporary housing, and as with everything the bigger the better! Within the month he'll be re-housed in my new 46 gallon reef tank.
-Diana