I'd think that just knowing the geometry of the frame wouldn't tell if you it fit you. The fork's length and bends, wheels, stem length and angle, seat post set back or being straight, bar bend and crank arm length all have a bearing on the bike's fit. A lot of people buy a bike on-line and are fine with the choice. As Barrett said, these are most likely experienced riders that know what is going to work for them in a bike's fit.
As pdlamb said:"Second, bikes that look identical on paper from the specs often don't feel identical. One of the best bits of advice is to buy what you like to ride, because then you're more likely to ride it. Again, if you buy after looking at a piece of paper or a web browser, the onus is all on you." When you buy at an LBS you are on the exact bike you will be riding. An LBS can fit you in all aspects as well as having any components you need changed.
Hope you find a good bike that fits you properly.
Bill
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Semper Fi, USMC, 1975-1977
I Can Do All Things Through Him, Who Gives Me Strength. Philippians 4:13