Correct stirrup length is an important part of the overall picture. Stirrups that are too long make for unstable lower legs, and tend to put the rider in a “chair seat” (which really looks like they are sitting in a chair), with legs too far forward and always behind the motion of the horse. Stirrups that are too short also affects the riders position, often making them cramped and tip forward. Stirrups that are juuust right will allow the rider to keep their heel under the center of balance and in line with their shoulder and hip.
The length of stirrup you choose may vary slightly depending on what you are doing. A more novice rider can benefit from having their stirrups shortened a hole (or two if they’re small;) ) to help them stabilize their lower leg, and same if you are planning on jumping.
Standing next to your horse, hold one hand up to the top of the stirrup leather. With your other hand, hold the stirrup iron up to your armpit. This should be very close if not exactly the length you need.
Let your legs hang down while mounted; the bottom of the stirrup should fall at the ankle bone.
The optimum angle on the leg is around 120 degrees between the femur and the tibia.
They must be even! Do not rely on a friend standing in front of the horse and eyeing them. There are too many things that can play tricks on your eyes- the saddle may be a bit off center, the horse not standing square, or rider sitting off center. Measure, from the flap on an english, and fender on a western. Use string, a tape measure, or the width of your fingers, whatever is at hand. Leather stretches!
The heel should be the lowest part of the body whatever level or discipline you ride.
Stirrup meeting the ankle bone
Check out the angle of the riders leg pictured above,
and how when you tilt your head a little bit, the rider below is about the same. If that angle works for a bull rider, surely it should help the rest of us stay on!