basically a horse that is very relaxed and bombproof. they dont like them too big, from 13.2hh to genrally 15.hh, but will take the exceptional one that maybe a bit bigger. We had a horse in the RS who used to go during winters to a DRC, he would normally have had the winter off, but he preferred to work. He was 16.1hh, but totally safe in every respect.. i think it also depends on their size, width etc... if too wide then the disabled person may not have the balance to be able to sit on the horse safely.