First, let me second everyone's opinion about getting the attitude under control -- but you may find that once you've addressed this, that forward, confident personality can serve him well as a driving horse. My gelding is still Mr. Attitude in hand, but just show him a cart and he snaps to attention. Driving is HIS JOB and he takes it ever so seriously.
If you're somewhere around 5'4" I'd guess that he's somewhere around 36". (Based upon the fact that I'm 5'4" and my 38 inch mini comes to my waist. Keep in mind that silly as it is, minis are measured to the last mane hair rather than the top of their withers.) I assume you're not looking to measure him for show or registry purposes, so you can get a rough idea with a yardstick and a triangle.
Numerous friends of mine compete successfully with smaller minis in Combined Driving Events, so don't underestimate his pulling power.
You have to also consider build. A sturdily built 36" mini can be a tank.
Next, you have to consider the terrain. On flat, hard surfaces, a mini of that size can easily pull two adults in an 85 lb. Easy entry cart. The same mini might have trouble in an arena with deep footing, uneven ground or hills with those same two people, but if one gets out, he will amaze you with his horsepower.
Then, of course, you have to condition them properly, just as with a big horse.
Just have one drag you across the corral -- or try to give my 38" mare a shot -- you'll see how strong they are!
As for carts, there most certainly are many choices for minis, but assuming you are looking at recreational driving, as opposed to the breed show ring, ADS shows or CDEs, a good place to start is with an easy entry. If you're comfortably funded, your rear end will thank you if you "spring" for something with better suspension, but you can start with something like a Frontier or CTM easy entry cart (and they hold their value pretty well). Just make certain that the cart you get is well-balanced, as you don't want more than about 10 lbs resting in the shaft loops.
Harnesses -- PLEASE don't think that because he's a mini you can cut corners on harnesses (not that I think you would...it just bears emphasizing). In my opinion, the best out there for recreational driving are the Chimacum or Camptown Sport Harnesses (also sold now as Comfy Fit Harnesses). These are not cheap, but they hold their value and are worth every penny. Ozark MiniTack and Country Carriage are also very good. There are other companies with quality harnesses, but don't expect to find them on Ebay
. If you don't object to butt ugly, Rons Presidential Harness is well-made and comfortable for the horse if not the eyes and is very reasonably-priced.
You can have so much fun driving your little guy AND you'll be giving him a job. Have fun!