That chart on the ADS site is a good reference.
I have a 50" pony (12.2h) and he uses a wooden meadowbrook cart with 36" wheels. When level, the shafts sit at 38". The shaft loops do of course adjust on your harness saddle, like stirrups on a riding saddle, but there’s only a few holes, and if your wheels on the cart are too short/tall, your shafts will still most likely end up tipping up or down, and it won’t be balanced.
For the length of the shafts, a general (but I’ve found to be very accurate) gauge of how long the shafts need to be is the same measurement as the horse’s blanket size. I’m not quite sure I understand why that works, but it does.
All of my horses have the exact same length shafts (from tip to singletree) as their blanket size, mini, pony, and horse included. And they have plenty of space between their tail and the single tree. I think somewhere around 10- 12", never actually had to measure, but if I had to guess. Maybe next time I get them hitched, I’ll measure that distance
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