Men's extreme gear hawaiian shirt size M or S?

In good pre-owned condition. Awesome, vibrant and colorful! Made of 100% polyester. SHIRT LOOKS SIZE SMALL....MEASURE YOURSELF BEFORE PURCHASE. Measurements: armpit to armpit 20" CHART I SEE SHOWING THIS BEING A SIZE S., front top button to bottom hem 23", front top button to bottom hem 23", front flat across bottom hem 20", buttoned collar 14". SEE LAST PHOTO....REINFORCEMENT SEWING SLIGHTLY UNDONE UNDER LEFT ARMPIT. UNDERNEATH SEWING IS SOLID. From a smoke free and cat/dog friendly home. (8)