How do you know how high your horse can jump?

That's a hilarious theory. It depends on the horse's fitness and attitude. A horse can have a perfect jumping conformation and breeding but if it doesn't like jumping, it won't try. I've seen plenty of cobs that are amazing little showjumpers. The only way of knowing is testing them. Loose jumping or jumping on the lunge is a good way to see how the horse jumps, and also gives you a chance to put the fence up a bit to see how they cope. Also, if a horse has very springy paces and a pingy jump, they're likely to be able to tackle big jumps. A horse that throws itself over jumps, hangs it's legs or doesn't have a 'spring' won't be as good.

Where do people get these theories from! As said there are plenty of small ponies out there who will easily out jump a much bigger horse. Free jumping can help to give an indication of a horse's ability, but obviously most can jump higher when without the rider or ridden over a single fence. Gridwork can be helpful for a number of reasons, but it's often a way to introduce larger fences as well as helping with rhythm, balance and striding. When it comes to course start off with a small course that you feel comfortable with (60-70) is typical mini height and see how he gets on. If he's jumping well then you can slowly start to adjust the height and also the width of some of the fences in the course (rather than putting them all up higher at once)to see how he copes with the difference. If he's struggling then you'll know he's reached his limit.

Free jumping your horse is one method of telling how high they can jump. The jumps at liberty in an arena with jumps setup down an alleyway style.

There is no way of telling how high a horse can jump, it all depends on them. If what your friend said was true then all the olympic horses would be 18hh+. As for your horse, just because he jumped one jump cleanly it doesn't automatically mean he can do it over and over again, especially not on a course. If you wanted to start competing him or jumping courses start at a height that is low that you KNOW you won't have any trouble with, if you get a good, clear round move up. As you get higher make it so you have to do say 3 competitions or rounds before moving up again, when you see him start to struggle you know you should stay at that height and work on it. He may be able to go higher with some practice, he may not. Just start off low and work your way up slowly, and as for injuring him, horses can injure themselves doing ANYTHING. As long as you are confident and certain he can do it there is no need to worry, don't do anything you are uncertain about!

Honestly, just trial and error. You don't want to push a horse that isn't fit to jump too high. If your horse is sound, fit, and healthy, set up two standards in an arena. Just a regular jump, no oxers or anything funny. Space the oxers far enough apart that your horse has room to recooperate. Have a trainer or someone that you trust in the arena with you and go over the jumps. About every two times you make it over okay, raise the bar about 3-6 inches. Just don't over work him :) About the leg length thing, I don't think that's true. Teddy, Karen O'Connor's event horse was like 14.2 hands I want to say and he was jumping way over four and five feet. I think it changes from horse to horse. Good luck! I hope you find out how high he can jump!