I have limited experience -- but enough to be suspicious of manufacturer's measurements...

I was looking at bikes this past August and rode a number of 17.5 inch bikes -- and they all felt a little different.

But, what really struck me was the difference between the 17.5 inch Trek DS: the 2012 model is simply a lot bigger than the 2013 model. I say that mostly because of the feel and partly because of the actual specs (the bottom bracket it higher on the 2012). But subjectively: the 17.5 inch 2012 feels more comparable to the 19 inch 2013 model than it does to the 17.5 model.

So, if you can, ride it first to see what fits. The specs can get you close -- but only your butt knows for sure!