The sand should be twice as deep as your biggest crab. So if you have a normal smallish hermie, three inches is fine. The larger they get, the more sand you should have in there, unless you're going to use a separate isolation tank.
If you use a separate isolation tank for molting, then you need to keep a close eye on the hermie and when he starts digging, you move him into the isolation tank.
That's what I do (have separate isolation tanks). My jumbo isolation tank has about 18 inches of playsand in it, but that's because my jumbos are all the way up to softball-sized!
![Smile :)](./images/smilies/icon_smile.gif)
Plus I like to spoil my hermies.
But in general for a new crabber, I'd say 3 to 4 inches is fine.
happy crabbing!
Christa