The shirts are actually larger for the same size than other shirts (my medium shirt is like a large in a different brand) and since it is a light material, you can move around easily. The material has zero stretch so you do not want to go too small. If you wear a medium, buy a medium. Large, buy a large. Because of the ventilation, you do not want the shirt to be as fitting as a regular t-shirt, though they're not baggy.
{edit} I'm not suggesting these shirts for having logos put on. I just realized it will make a great shirt for working on my car during the sauna summers we have.