They get 10-14 inches, males larger than females, most of them are hybrids with midas those (amphilophus citrinellum), males get a large nuchal hump, absolute minimum tank size for a single RD is 55 gallon, but since they get larger than the width of a 55 a 75 is really better, with tankmates a 125 or larger would be minimum IMO because they are extremely aggressive toward their tankmates (even in a 125 it might kill every other fish), they are very aggressive. They are very hardy fish though, water pH probably won't be a problem as long as you are keeping other fish alive in your water without a problem. Just keep up on the water changes and when its big make sure it gets enough dissolved oxygen with an airstone or a powerhead.