We found this article from www.horsechannel.com very helpful:
A: Stirrup length, as it turns out, is a very variable and personal choice. Some riders feel more secure with stirrups that would feel jockey-length to other riders.
As a general starting point, take your feet out of the stirrups after you’ve warmed up your horse a bit and allow your legs to stretch down around his barrel. The most commonly cited guideline is to have the bottom of the stirrup hit mid-ankle bone for jumping small jumps.
If you are going to be focusing on flatwork, you may be more comfortable in longer stirrups, adjusted to just below your ankle bone. If you jump larger fences or feel like you are reaching for your stirrups over fences, try adjusting them to the top of your ankle bone.
The most extreme examples of stirrup length include high-level dressage riders, who rarely post the trot and want their legs hanging nearly as long as they would without stirrups. On the opposite end would be eventers riding cross-country, especially over very big jumps. To stay up off their horses’ backs for long stretches of high-speed galloping, it’s best to ride with a short stirrup. Their saddles have very forward knee rolls to accommodate this length.
While you may have heard rules such as “go up one/two holes between flatwork and jumping,” these generalities are fairly useless due to the varied distances between holes in different stirrup leathers. Also, consider the fact that a 5-foot rider will have much less shortening to do to get her stirrups to jumping length than a 6-foot rider would; it could be the difference between one hole or six.
Finally, consider the type of horse you are riding. On a very round-barreled horse, you may want longer stirrups to feel that your legs are wrapped around his barrel for security. For a very narrow horse, you might feel better in shorter stirrups.
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