2 Dogs is right about the height location for measuring. Mini Assococcasion using last mane hair Is different than any other equine measurers.
What amount of work do you expect from him? Even on big horses, we try not to overload them in serious work. You want to have his total load less than 80% of his body weight. Leaves you some spare so horse stays in control of his load, not always fully exerted.
Mini folks donāt seem to take that into consideration. Cart, passenger, weigh as much (or more) as the equine and still is not called out for being overloaded.
I would look at bigger animals, measured at the withers, if you really want to drive very far or longer times, go down the road some with him. My friend drove a 36 inch one who was a determined character, when faced with harder going. Other minis would just quit. She went to a Pair pulling a 4-wheeled vehicle, because it was easier on him. She was not heavy, nor was his cart, but heavy ground, uphill work, sandy trails were just too much for only one small animal. For ring showing only, a mini might not be working so hard, so a bit smaller might manage for that kind of work.
Deep going, like a western riding ring might be impossible for in is to pull in. Saw that happen with very slender girls driving.