I always buy the 100% polyester shirts because they breath more than other shirts do.
I am a big guy and have a hard time with shirts fitting. I bought the 3xlt and it was actually a 3xlt. Very comfortable and seems to be holding up very well.
The NordicTrack Men's Big & Tall Performance Polo is a really amazing quality polo shirt. It is made of great knit fabric and for the price you cannot get any better.