The 'standard' for Frontier and several other EE pipe carts is 48" for A sized; you can order a 'set' of 54" for B sized. I have used the 48" on all of my A driving horses, 32-34". I do know of someone who cut a couple of inches off of her Frontier 48" shafts, on the suggestion of an ADS clinician; they were still OK, though just a BIT short for my preference. I got a set of the 54" shafts for my fully-38" B mare; they were just right, IMO. Based on my experience, I'd say the two lengths are suitable for each height range.
The shafts on such carts are essentially 'straight' shafts, which are supposed to come just 'to' the point of the shoulder when hitched. The Pacific and several other more 'upscale' carts have more of a 'gig' type shaft(and sometimes, a true 'closed end' marathon shaft.) The marathon shafts are only supposed to come to the location of the harness saddle/tug loops, and the 'gooseneck' upward/outward curve of the gig type shafts means that they by design do not properly come to the point of the shoulder, meaning that they *could be* shorter that straight shafts and still be of a proper length.
It appears that the Smart Cart pictured with the Whiteman's lovely Levi is still suitable to his size; however, I would judge that he is about at tall a pony as it would work for...IOW,I do not see it being suitable for a 46"/vicinity pony, at least the cart as pictured. If yet larger wheels, a wider axle, and longer shafts were put on(and I have no idea whether or not that is a possibility, as I don't know the sizes/lengths of those parts of the (prototype)cart pictured), then perhaps it could be used for a pony another hand taller? When you consider a 36" to 42" height range, it would appear that the Smart Cart would probably be quite suitable.
Margo