Id say at would be a nice size, Males dont get considerably larger than females.
As long as you have a nice sand substrate they should be fine. Remember though that they would prefer a neutral pH as oppose to the softer water of most Geophagus. The "Red hump" geophagus which is comprised of 3 species are a very northenly species so a harder water (pH 7 - 7.2) would probably be better.
Dan