There's also the aspect of finding "what could work for me?" Practically, at an LBS you ideally get steered to what the manuf recommends for your body size (measured somehow). There also some detailed guides on calculating the "ideal" geo, and these are mainly for road bikes. and for understanding what to think about in sizing.
Good non-quantitative articles include the one mentioned above, by Dave Moulton. Also there is the one written by Peter White, on his shop's website, and the websites (or are they blogs?) by Steve Hogg in Australia.