When I asked this question awhile back on another board Jason recommended 65" shafts--he posted pix of some of his ponies driving in 65" shafts (show carts) and they looked good. That is what I got on my pony cart this year & lengthwise they are great--unfortunately I do have find someone to make a different set of shafts for this cart because it turns out these are wider than what was listed on the website, and are much too wide even on my widest pony.
So, I would say 65" but just double check the width of the shafts before buying. My cart was supposed to have 23" width & came with 24"--I would say that 20" would be good, 21" would probably work.