The shirts have a generous, oversized fit, with a 100%, rugged heavyweight cotton that has only a small amount of stretch to it. Here’s what that means:

• If you’re ~6ft. and a bit thicker (~250lbs): size XL has a generous fit. Size L is a more tailored fit. Both could work.
• If you’re ~6ft. and a bit leaner (~170 to ~190lbs): size XL would be a baggy fit. Size L would fit comfortably. The shirt fits generously, and the sleeves are a bit longer. If you don’t like that, Size M would be a better choice.
• If you’re 6ft+ and a bit thicker: Size XL – 2XL might be better. If you wear a size 3XL normally, a size 2XL should fit you good. 
• If you’re 6ft+ and a bit leaner: Size L would fit great. Size XL might be better if you have broad shoulders, or if you want the shirt to fit a bit longer.
• If you usually wear a size S: If you’re taller, you might prefer a size M. The shirt has a boxier fit, so you want it to be long enough. If you’re shorter, a size S should be fine.