5 in a 55 will work you're never going to have dinner plate size fish in there anyway unless you do daily 75% water changes and being practical that's never going to happen. 5 to 6" dia fish are a good size and is what can reasonalby be expected. Try sticking to some of the tank breed species that don't grow as large Yellow face Mellon, ghost nebulas, bulldogs off the top of my head don't grow as big as some of the snake skins for example. Use Vals (cork screw is nice) planted along the back wall of the tank they grow on runners so you can cut the runners and replant in open areas they also get tall so they will cover the top of the tank and difues some light, Discus love to swim through them.