Erg, how irritating is it when designers don't provide a size chart?
But here are the results of my investigation. The sleeveless version is modeled in a size S here, which they say is size 2-4. Also, when using the Saks fit predictor on another button front silk blouse, they suggest I take a size 2 in Rebecca Minkoff (I predicted my fit based on Equipment, Joie, and Bailey 44). So my guess is to take the smaller range of your true size.
Now... I sure wish I knew what the color looked like IRL! Some citron is awesome on me, but the florescent kind looks weird...