I followed everyone's advice in previous comments and purchased a size XL for my thin husband, who normally wears a medium.  My husband is 6 foot, about 165 pounds, and it fit him perfectly.  Cute ref shirt made of cheap, thin, polyester fabric.  He wore it to my son's sports-themed birthday party, so it was perfect.  However, do not purchase this shirt if you want a shirt made of quality, thick fabric. This is strictly a costume-style referee shirt.  All in all, we are happy with it.  We even ironed on our last name onto the back and it took really well.