In reply to mrdigitaljedi:
Wasn't aware of any "road bike sizing" - afaik this is the same measurement, but expressed in cm rather than inches (divide by 2.54)
If you mean the tendency to provide only a XS, S, M, L, XL size options then generally Medium would equate to 17.5-19" frame size, but can vary a bit by manufacturer (especially on Full SUS due to the myriad of frame designs)