Also a good method to find about how long your stirrups should be is by standing on one side of the horse get an arms length between you and your horse(you should have your longest finger just barely touching the saddle) and take the iron bar, and you'll know if it's a good length if the iron bar reaches your underarm/shoulder. I've used it for my western and English saddles for years and it's worked.
This is good advice, but it creates a problem. English leathers (the ones made OUT of leather) stretch. Most people mount on the near (left hand) side of the horse, and that leather stretches first. After awhile your left stirrup leather will become longer and uneven. Most English riders simply switch sides of the stirrups and their leathers. Another thing to do is to often mount your horse on the off, or right hand side, to keep your leathers even. Be sure to train your horse to do this, since they physically need to understand all skills on each side of their body. The only reason that we mount on the left side is that soldiers (from the Romans on) carried a sword inside a scabbard which hung on the left side of their belt. It was impossible to mount the horse from the right hand side with the scabbard, and even more difficult since they mounted/rode without any stirrups (a Mongolian invention.) In addition, the comment about measuring from your armpit to the stirrup bar is correct, as well as using your ankle bone to fit just above the stirrup, as a standard, average length. Please remember that the arm measurement does not work on a Western saddle, and that there are other saddles that people ride where this adjustment will not help you. My DH and I ride McClellan saddles, and I cannot measure my stirrup length by using my arm.
I had this same problem but when I was just flatting around the arena I had my stirrups a little bit longer, but when I jump I shorten my stirrups a hole or two. If they still uneven try tightening up your girth. You can also try to punch new holes into your stirrup leathers to make them feel more even.