I'd agree with P44, a 5x2' footprint would be best if you want to grow the fish out to their maximum potential, could probably get away with 20" height but 2' would be better. I know people often recommend smaller, IMO a fish with the potential to get that big needs more room. Maybe a 4x2x2' would do for a pair, at the very least.
There was a monstorous oscar at Animates in Mt Eden a few weeks back, in the hex tanks where there plants were (and corals once upon a time). Was the biggest I've ever seen, at least a solid 12" by my conservative guesstimate, maybe bigger. Anyone who says they can be kept in a 55g has obviously never seen a fish like that. And as for Animates little info card on the tank with the small oscars saying "15cm and 165L".... :evil: :roll: