None.
Anemones are very difficult to keep in small tanks. A 30 would be the smallest I would recommend for keeping an anemone. They require pristine water conditions.....difficult to maintain in a nano tank.
As for hosting an ocellarius, they are most likely tank bred so many will never take to an anemone for whatever reason so even if you did get the right anemone, they may ignore it completely!
Im going to try to keep one in a 20 G long tank. It is possible, but you have to be VERY prestine about your water conditions, and you must be sure that you have all of the nessesary equipment...
I havent decided yet. Probably something small, which is the way you should probably go also. Are you going to have a host? There are some obvios candidates that i would definetly say no to, carpets, anything else big, etc...lol