I have a show cart with, I believe 24" wheels, and 48" shafts for a big moving 36" horse. That shaft length seems to work ok with him in a show ring environment.
My marathon cart, however has adjustable shafts, that I keep set at 54", and I wouldn't want them any shorter, lol! I use it for everyday driving too, and, even though we do a lot of cross country and road driving at sane speeds, I still like that extra room for comforts sake, because they move a lot bigger when they're going somewhere aside from around-and-around, lol.