When you go to purchase any of the items on this list of best Harry Potter shirts, the manufacturers provide a size chart. Use this chart to take the right measurements for yourself or the intended wearer. You want to get as close to this individual sizing chart as possible. After that, it comes down to personal preference. I usually straddle the line between two sizes, and I order up in that case so that I can comfortably wear a shirt without it sticking to me like plaster.