I would be surprised at a tank for one, most use the pop up or build indoor ponds for stuff that big, or outdoor depending on where you are located or if heating is an option. It's no scientific measurement, but I always want enough room that at full grown if the fish spooks it can go into that glide they do after and have enough time to slow down comfortably before hitting the other end of the tank. Usually gives the fish enough space to cruise, and also in my own opinion cuts down on the likely hood to them hurting themselves...that said though my motto has always been 'Whats the biggest tank I can go with in that spot?' then plan the stock list accordingly. 
