T-Shirts (and clothing altogether), tends to vary size wise regardless of whether it's tagged XS or XL, so we provide p2p (pit to pit) and t2b (top to bottom) measurements for each of our items.
The easiest way to see if an item will fit is to take an similar item from your own closet that fits you comfortably IE: a shirt when measuring a shirt, or a jacket when measuring a jacket, and compare the measurements of your item with the item you are interested in. This ensures that you will have an accurate picture of how an item will fit on you