Depends what you are planning with the planting I guess. If you aim to fill the tank with large plants then of course your taking space away from the tank. Depending on the dimensions of the tank I would consider 40G a minimum size, a group of 5 discus would work in a bare bottom tank with a few pieces of bogwood that could be planted. I would start with fish around 4" and over to make life eaiser.
I guess this is your first time keeping discus and so a larger tank would be slightly easier, but if you keep on top of the of it it would out ok. Just be sure to research them fully and be aware of the water requirements the fish require.