For a scobina or histrix if you can find them, a standard 180 would be fine for life. A retic would be fine in one as well but only if it doesn't have a full tail. If it does you will want to go wider, like at least 30in.
If by chance you do end up with a motoro or something a tank of 4' wide would be optimal, but they would probably do fine in a 3' wide tank.