Sizes stuff is crazy! Usually European tshirts are smaller than American, but it always depends on the brand. Usually all the sites have a size chart, which is the best way to know, comparing with one that you already have and fits you.
If you are skinny, you can always "reconstruct" the tshirt, make someone reduce it for you (or yourself if you are a sewing master!). Here you can find good examples:
http://www.craftster.org/forum/index.php?board=348.0 , hope it's useful