Well my horse jumped 5' in his day. He is old now.
He has jumped 4'6" on his own since we have had him.
Him and I have jumped 2'9" at the highest but hope to get higher once I get more confident and he gets more muscle.
I jumped for the first time ever in my life on him last June, so have only been jumping for a year, had a 3 months break because he dropped too much weight though.
My mare was trained up to Grand Prix, highest I've jumped her and known the exact height was 5' to 5'3", but she has gone and jumped (with me on her back) the standards themselves that were set up as an oxer. That was one of the first few times I jumped her, and the actual oxer was set up at about 3'3", and she decided to try and run out too late, and had to jump the standards. Not exactly my most shining moment lol. I've consistently jumped 3'6" to 3'9" in pony club though.