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For pleasure riding in a Western Saddle, your stirrups should be longer than English (with the exception of several of Dressage Instructors that prefer a VERY LONG LEG).
Measure the same as you would with English. Place the "beefy" part of your palm on the outside of the stirrup bar area, and pull the stirrup skirt down or out towards your armpit--making sure to keep the skirt strait, smooth and stretched as much as is possible. The bottom of the stirrup should reach the bottom of your armpit area.
Once in the saddle, stand in the stirrups. You should be able to place your hand between your crotch and the saddle. The width here is a personal preference.
To sit deep, secure and comfortably with your heels down, and toes up and maintain contact with the horse at all times, you will want to have no more than a 3" space. The greater the security of the lower leg, the greater the rider will have in maintaining position at all gaits.
The lower leg and rider control is lost if the stirrups are too long, and the rider is thrown out of the saddle at rough, uneven gaits (such as at a gallop) if the stirrup is too short.