Sizes vary a little between garment brands, so every product page has a Size Guide button with the exact measurements for that item. As a general reference for our women’s tees (garment width measured flat, armpit to armpit; length from the highest point of the shoulder):

Size Width (in) Length (in) US Size
S 16–17 25–26 4–6
M 17–18 26–27 8–10
L 18–19.5 27–28 12–14
XL 19.5–21 28–29 16–18
2XL 21–23 29–30 20

Tip: lay a favorite tee flat and measure it — comparing those numbers to the chart beats guessing every time. Between sizes? Size up for a relaxed fit, down for a fitted look.