The shirt is made of silky polyester, oops, wait, my mistake...
The TAG STATES that the shirt is made of: "Luxurious 100% Texturized Nylon Knit"
-- so it would definitely stretch a little for a form fitting silhouette, but lying flat, it measures...
shoulder seam to shoulder seam: 18.5 inches
underarm to underarm: 21 inches
underarm to cuff: 19.5 inches
top of the collar in back to hem: 30 inches