Typically I order a large if the shirts were made of cotton, taking into account the shrinkage. Because this one is nylon and lycra, I took a chance on the medium and it is a perfect fit. There is no shrinkage when washed. They also allow wicking away of any moisture that accumulates. This one should last a very long time.
The shirt worked great for my first triathlon. Unfortunately I waited to long to order it so it didn't arrive until two days before. Normally I wear Large or XL, so I ordered a Large. It felt like a XXL. It worked, did what it said it would do, and it was cheap. Overall I was happy with it and will wear it again.