The rule of thumb for discus is 15 gallons per fish.
Discus can vary in size from 6-11 " as adults..one of the reasons some discus get so big is the way they are fed..they are fed a lot when young (to get them round)
there are different thoughts as to whether this is a good thing.
but they do get relatively large if you consider a 6" plate.
Another issue to consider is..discus generally are social and do best in groups the smallest being 4-5.
you can keep a mated pair in a 29 bare bottom with daily water changes.
but if you want a discus display tank..you'll need a larger tank.