Hunt seat riders' stirrups should reach just below the ankle bones. There can be some variation up or down depending on the type of riding, the style of saddle and the rider's physical build. A rider who anticipates taking fences may wish to have the irons higher. Stirrup irons should fit your boots. Small stirrups can tangle your feet.
Large irons are clumsy and difficult to keep correctly positioned.
Changing stirrup length is easy once you have mounted. Keep your feet in the stirrups, and one hand on the reins. Bend the upper part of your leg away from the saddle, and with your free hand unbuckle the stirrup leather, rebuckling it in the desired position. Pull the stirrup leather buckle up as far as possible toward the stirrup bar on the saddle. Correct length of stirrup leathers helps you maintain correct leg and boot position.