Just out of curiosity, is there like a general rule of thumb for how high a horse can jump relative to his size before there is a large risk of injury? Like how high could you say my 15 hh quarter horse could jump?
It's all about the individual horse. Hickstead was only 16 hands. He was also an accident baby, his daddy jumped into his mother's pen at a show and the result was one of the greatest jumping horses of our time. Teddy O Connor was only 14'2 and he Evented at the 4**** level. If the horse has the basic conformation that allows for good jumping, and a great heart, they can basically do anything. But you have to know your horse and know what it's comfortable with. :)
As long as the horse has both the conformation and the heart to do it, I would say how tall the horse is shouldn't matter. I do believe their heart/will is just as important as their conformation for them to actually jump high heights (as in, not just once in a while, but competitively/ constantly).
My 14 hand paint used to jump a 5 foot fence daily at his old trainers, he would jump out, eat grass, and then jump back in when he heard them coming over (they caught him by looking out the window, lol). He also jumped a five foot tire to get out of being lounged. But the point is, he's not ideally conformed for it, I would not make him do it constantly as a career because of that, but when he was determined to get over the fences, he did. I doubt he'd hold up jumping five feet as a career, but he can jump five feet when he wants to. I guess you could say they need a body that can carry their heart and a heart that can carry their body...