The smallest I know of is the Kole, as mentioned. They're not the most beautiful, but you wont have much of an algae problem
Scopas do ok in smaller tanks if they are long, like a 55 or min 40 long. Pretty easy if you get a 2" one. Bruised leaf lettuce or spinach sticking up from the bottom of the tank is one of thier favorites. Let them finish cleaning the algae out of the tank first.
they are one of my favorites, way prettier settled in at home than at the store. We used to call them Mink Tang. Look awesome around colorful live rock.