How do you know your stirrup length is right?

So... I have a problem with bracing off my right leg when at the canter, and since they say problems at slower speeds are just multiplied when you go faster, I'm thinking that I brace all the time, and just notice it mostly at the canter. I know that I have a problems with keeping weight in my heels, and its something that I'm working to correct, but I'm thinking that I may have some help from my stirrups too. More specifically, I'm wondering if my right one is maybe too short, even though I ride with both in the same hole.
My right leg feels ever so slightly shorter even just standing still, but how often is it that people have to ride in uneven stirrups, and would it really make that much of a difference if one of my stirrups was too short? Knowing that I have issues with getting the right seat, is the stirrup likely to be a red herring, and I just need to keep working on my seat?

When you say you are bracing into the stirrup, what do you mean? YOu are locking your knee and pushing the stirrup forward, like it's a wall and you are pushing into it? Are you riding English or Western?

It always helps to ride a bit without stirups to get that feeling of how your leg should drape down. Or, try putting them down 3 holes, then ride, raise, ride raise to the place that feels good, but give it a few minutes at each position so you can adjust before deciding if you truly need to go up another hole.

I often feel my rifht leg is shorter. But I dont' feel that I am bracing into my stirrups and when I post, they feel pretty even. I no longer lose my stirrups at the canter. That took years to get over. I was gripping up with my heels. Very bad habit.

Just because you have both stirrups on the same hole, doesn't mean they are the same length. Saddles can be made unsymmetrical .... Best thing to do is have someone stand behind (or in front) or your horse while you are mounted and look to see if your feet/stirrups are even.

Also, it is possible that you sit slightly crooked in your saddle OR your saddle sits slightly crooked on your horse. Again, have someone with a good eye observe you from the ground while you are on your horse.

So check those things at first, in regard to how you feel like you brace more with your right leg.

Ditto to tinyliny on the stirrup length. When you let your leg hang down naturally, the bottom of your stirrup should be about level with your heel bone (on the side of your foot) if you are riding western. At least that's a good place to start and try. If you are riding English, well it depends which discipline (jumping, trail riding, dressage, etc)

Bareback riding and/or riding without stirrups can really teach you to have proper leg position and keep those heels down! And it also helps you to learn to be less reliant on your stirrups.

I would lengthen your stirrup out until you feel comfortable (even get a punch & put a new hole in so that you feel centered & even). Also, how short do you wear your stirrups. If western, longer is generally better I find as it allows you to lean back more comfortably. Typically, you want to be able to fit 2-3 hands layed flat upon eachother under your butt when standing in a western.
Also, if you hold onto the horn at all when running in a western, it will mess with your balance & your one leg will be stiff & the other will flop around uncontrolably.

I ride western tinyliny. When I say that I brace, I meant that I lock my leg underneath me, and end up with my weight unevenly held on my right or left side. I feel uneven when I canter, so I know somehow I'm off balance. My instructor says I try too hard to keep my heels down, and that that causes me to brace against my leg, causing me to bounce around when cantering. I brace my right, and the left leg either ends up carrying too much weight because I'm trying too hard to to balance out, or my foot ends up coming out of the stirrup because most of my weight is on my right side. I know there is plenty of human error in the equation, I'm just trying to figure out if I've got a mechanical problem too.

My canter has improved a lot from where I began. At first I could only go a few strides before my feet were out of the stirrups or I was so off balance I had to stop. I usually had to pull my saddle back to the center too. Now my feet more or less stay where they should be, and I actually managed to get two and a half laps at the canter in before I had to stop and readjust yesterday (which was a first, usually I get in one). I usually don't have to adjust the saddle anymore either.

Does that help explain it any better, or just make it more confusing? I will ask about doing work without stirrups next lesson- its something that I've been thinking about trying for a few weeks.

No holding on here! I do have a tendency to hold my hand up (the free one, not the one holding the reins) when cantering, but its something that I'm working on. Physically altering the saddle is not an option, because I ride in my instructor's saddle (and on her horse), but I my ask to try the saddle with uneven stirrups next lesson. The length of the stirrups, well every lesson it has to be evaluated, because multiple people use the saddle, so its always possible that they may get too short or long. Usually if they don't feel right I just stop and ask to have them adjusted- now I just wonder if, you know, I may need to play with it a bit more.

In western saddles they say to measure holding the stirrup out towards you. It should roughly be basically the length of your arm all the way to your armpit. However, I like mine a bit shorter; there is no right wrong, just what makes you feel comfortable. (though people will tell you that you're wrong probably!) Ignore it and do what feels best for you unless showing.

Just another thought, it could also be how your horse travels. I am perfectly balanced on almost any horse, but the few times that I have loped on our old 'kid' horse, I was miserable. It's not that he is unbalanced or anything, he just travels funky and throws you off balance to the right, regardless of which lead he's on.

I agree with letting out your stirrups and then slowly taking them back up until they are comfortable. Not all saddles will be exactly the same length even on the same hole or your legs may be slightly different lengths.

One other thing you might look into is seeing a chiropractor for yourself. When I am out of place in my lower back and hips, I end up having one leg longer than the other in the saddle and I have to re-adjust my stirrups until I can get back to the chiro and get it fixed.

Here are a couple good ways to figure out what your stirrup length should be. I would personally go with the ankle one though. Here is the link :
How to Get the Correct Stirrup Length

I was always taught, that they are 'right' if you can put a fist like an ice cream cone under your butt when you stand in the saddle. And, usually if you are on the ground and put the stirrup in your armpit, you can touch the bottom of the seat area. Another one is to let your feet drop out of the stirrups when you're sitting on the horse. Put your leg straight down. The bottom of the stirrup should be even with your anklebone.