Jill has it right, IMO(and, with all due respect, about the fit of the cart to the mare in the photo...) In my experience, the largely standard 48" shafts of most of the EE carts are right for most larger A sized horses, and *might* work for small to mid-sized B's, depending on the individual horse's body build. Remember that the shaft tips should come just even with the point of the horse's shoulder, for proper fit-and that is dictated by both shaft length and proper trace length/adjustment. When the horse is actually"in draught"(draft)(-meaning actually engaging the vehicle via the breastcollar or neckcollar), the shafts tips may "end up" SLIGHTLY behind the point of the shoulder, but it should not be by much.) The Jerald show carts come in 46" and 49" shafts-the longer ones would likely also fit *some* B's, though I have seen people driving larger B's to them, and the shafts were simply too short. Frontier offers a longer(54")set of shafts,which can be easily interchanged with the standard-length ones(or, I imagine, could be ordered as the primary shafts, should you be planning to drive the taller, B, horses)-- I ordered a set several years ago, and now am using them for my 38" mare. My road cart has 48" shafts(wooden)-though I have driven the 38" mare to it, the shafts are truly too short for her, and I wouldn't hitch her to it again unless I decide to have the shafts replaced with longer ones. (As I am also pretty much out of showing, I *may* just sell the road cart-depends on how versatile the Bennington is).
Since I have the 54"ers on the Frontier, I have had occasion to also hook the cart to my 34""Melody", when she was being "teacher" to a driving student-it worked fine for the basic, in-the-level-arena work, but put the cart a bit further from the horse than is ideal(where it would be less manueverable, for one thing.)Currently, I am getting close to being ready to actually hook the cart to two horses I have in training-both are between 33-34"-and will need to change the shafts back to the 48"ers when I do so, for optimum correct fit for these green horses.