Mine is currently in a nano tank (10 gallon), BUT it is still very young (less than 4 months?). I do have a 60 gallon that I've setting up for it, which is the refugium for my 200 gallon reef. The only reason he/she hasn't been moved is a mysterious die off of fish from the main 200 gallon tank, so until I find out what the problem is I 'd rather take the risk of the small tank than the mystery tank.
The nano was set up just for him/her, and knowing that a 10 gallon is too small, I set it up with a filter that was rated for a 30-70 gallon tank. This made me feel better with the smaller tank. The tank itself has 2-3lbs of live rock and 3 different sizes of PVC elbows and Tees for him/her to hide in. The only time I've seen 'em in there is when it drags a big fiddler down to davey jones' locker! LOL. Normally just hangs at the front watching the room. Very mellow as long as you keep your hands out of the tank, if they go in... well normally only food goes in so he comes looking for it! Thought for sure I was going to loose a finger last time! LOL, just kidding, but he isn't afraid of anything, never takes off or squirts.
SO, my OPINION (meaning that you should listem to those who've kept them longer than I) is that you can get away with a 10 gallon to start with as long as you put a BIG canister filter on the tank, which I hanked from an established tank (including the 10 gallons to fill the tank!). But I could just be lucky, don't follow my example unless you just HAVE to, you know?
Take it slow, they have a right to the best life that you can give them!