For me, the sizing on Adidas items has been extremely inconsistent. In cotton Ts, I usually run a medium or even a large. However, in the polyester performance type items, I usually run medium or even a small. I actually stopped by the bookstore on my way home yesterday and tried on one of those Iconic Training Hoodies. The medium I tried on fit me a bit large, but they didn't have a small for me to try on.
So, in general, I would say cotton type Ts and sweatshirts usually run true to a bit small, while performance type gear runs a bit large such that I have to order down a size (at least within the small to large size ranges).