I've always said a 75 is good for a long time but I prefer length but u need 18 inch width and probly bigger if it reaches 18 inches. But it will probly be the only fish u can keep in the tank as malas don't like tankmates usually
it would take a while so your tank is good enough for now, years down the road I would say upgrade to a 24'' width tank. sometimes mala's get along with other mala's and big cichlids that they can't eat. But this depends on the fish
20 is there max size in the wild, 16 seems to be a good size in the aquarium. A good foot print as wide as possible would b good. If you can get 4x2 LxW that would be great. A shoal of large SD's would add some movement to the tank.