Your stirrup length should allow your legs to hang naturally, with a slight bend in the knees when your feet are in the stirrups. For an English saddle, the stirrup leathers can be adjusted by moving the buckle up or down to reach the desired length. On a Western saddle, adjust the Blevins buckle to set the stirrup at the right height. A good test is to let your legs hang loose and check if the stirrup touches your ankle.