Epic, you could have asked an easier question, like what is the square root of three times the distance between Mars and Venus What make it so difficult is every jersey maker sizing is different. Right now I wear jerseys that are L, XL, XXL and XXXL depending on who makes them. Right now, our team uses Hincapie jerseys and their sizing seems to be pretty close to a couple of Giordana jerseys my wife uses. She just moved from their youth jerseys to their womens jerseys this year.
My advise would be to get the small if she likes things tight, (most of our girl cyclist do) or a medium if she likes them a little looser.
As a side note, my daughter may have a few jersey's she has outgrown if you are interested. Shoot me a PM.