The shirts I measured were under arm 6 to 6.5" and outside arm 10 to 11". I measured both T-shirts and woven shirts.
Just measured mu Hubby's Polo shirt and it is 6 1/2 inches under the arm and 10 inches from where the sleeve joins the shoulder. I would allow an inch hem as well.
Hope this helps.
Thank you, I think I really need a t-shirt measurement. A polo would fit tighter.