Would put in minimum of a 10 gallon. I had a little guy that was in a 3.5 (filtered) and got ick.. he now resides in a 10 gallon with a better filter and heater. He has been clear of ick for about a week or so now and is much more active. He lives by himself at the moment, but he lived in a tropical tank for a while before I got him a tank. He did very well with all of the other species.
--Edit--
They are almost blind, so any fancy tailed fish that he would confuse with another Betta is going to be in trouble. Just keep that in mind, if it's a fancy looking fish, keep them away from a Betta. If it's a $2 fish and you don't mind losing one, then you could try it. Just pray for the poor little $2 fish.