The Straight Arm Is A "1/2" Swing Training Aid that improves ball striking consistency by minimizing arm bend on the backswing and keeping the player's focus on the position of the body and leading arm through impact. It is available in 2 sizes. The small/medium will fit most juniors and adults. I (Dane) am 5'9" and 165lbs. and would use the small/medium. I could also use the large but it's a little bulky for me by comparison. Only get the large Straight Arm golf swing trainer if you have big arms.