If you buy a sweatshirt from one of the gift shops, try it on! I just got a Jamaica one from the CN Gift Shop and a logo one from the CN Botique. They run about 2 sizes too small. I got two mediums and should have gotten X-larges. We washed them in cold water, so I don't think they shrank during washing.
The T-shirts run fine. In 4 visits starting in 2006, all the t-shirts (probably 10) have been correctly sized.