I get nervous buying certain products (mainly shirts) through online stores, plainly because I can't try them on first. Certain brands under the Retrolicious line tend to run much smaller than you expect, and that normally has to do with the material being used. To be safe, I got an XL for this shirt and it's absolutely adorable, comfortable and not super tight.
I love this shirt; not to tight, not to loose. It's good for casual day to day office wear. It makes me feel just professional enough--and a little bit quirky.