How to Measure a Shirt: Short Sleeve Width
Lay the shirt on its back. Spread the shirt sleeve out flat such that the seam along the bottom of the sleeve is where the sleeve is folded. Place one end of the measuring tape on the bottom edge of the sleeve, an inch from the armpit. Measure up to the top of the sleeve such that the measuring tape is perpendicular to the top folded edge of the sleeve. Do not stretch the sleeve but be sure it is spread out flat.