Where would you buy the Ruby from? If there's an LBS you know you'll be buying from, you might be able to get them to take some measurements (inseam, arm, shoulder) and then help you figure out which will probably fit you best. Ideally they'd have a good fitter to help you. Or talk to them about your options if you order one size and then find that it's not right.

Also +1 on pinson2's suggestion. I have a vague recollection that two of Specialized women's road bikes share the same geometry, though I'm not sure which ones.