for just 2 discus and resulting fry for life? i would say 3ft long, 20" + high and 12" deep.
I thought about this myself lately, making a small tank with a pair of Discus and having it open topped, heavily planted with aquatic and semi aquatic plants with intertwining wood roots, deep sand bed and a myster at the top of the tank. to make it look like a sorta amazonian 'pond' or swampy still water with a slight hint of black water.
of course i'm to lazy to do it.