Join Date: Dec 2010
Location: Ann Arbor, MI
Bikes: 2010 Niner EMD, 2008 Surly Steamroller, 2007 Giant OCR.
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That's tough because there are several factors involved. Differing manufacturers label sizing differently, so that she may fit well on Brand A's Medium, while she may need a large in Brand B. Also, there are major geometry differences between Men's and Women's frames, so that she may be comfortable on a Men's Medium but cramped on a Women's medium.
My best reccomendation is to have her test ride the bikes in different sizes at the local shop and get their professional opinion (hopefully they have one, and aren't just trying to get a bike out the door). Also, most shops offer a professional fitting, where they can take measurements and size her to a frame.
If you are looking to purchase a bike online, it is probably a good idea to get the bike fitting at a local shop (most shops will charge for this service), and get the specifications from them. Use the specifications and check the frame geometery of the manufacturer to make sure you are getting the correct size. If this is too much of a hassle, just by locally and pay a little extra to get a bike that fits her. If it fits, she'll want to ride it more!