These t-shirts are a unisex fit. When somebody asks, "What size t-shirt do you wear?",
For women, you should wear a size down. (If you wear women's size medium, you should choose small).
For men, the measurements on this probably match up to your answer.
The material of the shirt is stretchy and body fitting. If you prefer a looser fit look, you may want to try out a bigger size.
Chest (Inches): S 34-36 | M 38-40 | L 42-44 | XL 46-48
100% premium-cotton, ~6 oz.