Males attain 4 inches, though some sources cite 6 inches as possible; females are slightly smaller. A 36-inch tank is preferable, but I will leave it for the cichlid members to be more precise. This is a fish that cannot have tankmates in this small a tank, just so you know. A pair is it.
Have you looked into the dwarf cichlids of South America? Many of these would work here, and dither fish (small characins or dwarf rasboras would be acceptable. And substrate fish like the dwarf species of cories, over sand and in soft to moderately soft water.