If you keep discus rather than going by 'he said/she said', things seem a lot different. I would happily keep discusin 55g tanks, though no smaller than that, even growing them on with exceptions of course.
I kept four discus very much happily in 55g, I found that as a group of 4 they were very happy, the more discus you have, the more potential there is for them to be unhappy with their tank mates and an unhappy discus is an expensive mistake. I got lucky because I had 5 tanks discus could live in and swapped them round regularly until I could settle on groups that were happy, would dread to think what its like for people who cant do this as if the discus dont get on, you have to rehome one or t'other, simple as. Likewise if they start breeding, you will need to rehome the extras if you find you have a pair as they wont tolerate other discus so well.
I had a tank with a breeding pair, a tank with three babies, a tank with a shoal of 8 and a tank with a shoal of 4 and of all of them, the shoal of 4 were the happiest.