American Apparel t-shirts run a little slimmer than your average shirt. If you are on the border between two sizes, you should order a size up. For an example, Joe wears a medium American Apparel shirt, but a small of most everything else. Expect your shirt to shrink slightly from normal washing and drying. Washing in cold water and line-drying will reduce the shrinking of your shirt to a minimum.
Note: All sizes are in inches