I had my four tank raised horses in a 29 gallon TALL aquarium. Horses dont need width as much as they do height, especially if you have a mated pair. If you do purchase a mated pair, inthe morning, after the lights come on they will swim towards one another, grasp tails, and circle the tank while rising in the water column. I had tons of fiji branching rock in my tank so that they could grab on to stuff, and also had a dense growth of Caulerpa, as it harbored the little copeopods and crustaceans that they ate and hunted all day. I loved my tank and my horses they are definately one of the coolest and most unique fish in the ocean. Whatever size you do plan on, make sure you get teh tallest one you can. Sometiems youll have to special order one, but the happiness of the animals far outweighs the extra cost!
good luck
jon