I have one but got it a couple of years ago. They run small and are form fitting. I usually wear a XS or S in most t-shirts and I bought a medium and it's perfect. (This was based on other comments at the time.)
Likewise, I bought a S in the STS and wish I would have bought a medium as the the body is okay but the sleeves are tight (and I don't have large arms!) Again it's form fitting. So, I would go up a size larger than you normally wear.
Jo