I would think it depends on the species of horse you're going to keep! I wouldn't want to keep dwarf ponies in a 55g aquarium, for instance -- you'd lose track of them in such a large tank.
That said, I'm a big fan of "bigger is better." You get more stability in the system in terms of chemical fluctuation. I have a 37g tank, and while I think it's great, it may not be for everyone. It requires weekly water changes to keep it in check, chemical testing every couple days, etc. If I was to do it over again I would start with an aqueon 65 gallon tank (36x18x24"). That would give me almost double the water volume, and allow me to keep an extra horse safely. Believe me, my 110g tank is a lot less work on a weekly basis than my 37g horse tank!