We're driving horses that are 36-38" tall on a CTM mini cart with 54" shafts and 20" wheels; I think you could order the cart with 24" wheels if you wanted. A Frontier cart with 55" shafts (I think that's the longer length they offer) would also work.
If you want something a bit more pricey, the Pacific Carriages Smart Cart is absolutely PERFECT for the B size horses. They have 26" wheels, with 30" wheels being optional.
Jerald show carts--I'm not familiar with them, but have seen them advertised with 52" shafts I think?