Very good advice Tom.
Yes, humans are even more variable than bikes.
Speaking of crit bikes, brings up another consideration when comparing frames, bottom bracket height (or drop if you prefer). The higher the bottom bracket from the ground the smaller the frame has to be to maintain stand over height and the other edge of this sword also brings with it more exposed seat post. Something to really consider when looking at some bikes intended for criterium, track or rough condition use.
By trial and error over a few decades I think I've got my personal likes and dislikes pretty much ironed out and can put a bike in the correct ball park for myself or at least see if it is possible with a tape measure and preferably a level.
I'm 5' 11 1/2" or so and can remember being 6', my regular riding bikes have seat tubes from 57cm to 58cm c/t with top tubes in the 56 to 58 range all of which I can ride comfortably until the power plant gets too tired.
You can juggle things a fair bit with bars, stems, saddle and seat post and get stuff to work. With fairly normal road bike geometry in my opinion cockpit length is probably the most critical thing as long as you can get the bars where you want them and the saddle isn't sitting on the top tube or up so high that it be comes a wheelie bike.
I have a rather strange bike with a 53cm seat tube and a 58cm top tube that actually works surprising pretty well for rides of about 40km. I can think of a few frames I own that fit within my regular sizing/geometry range, but don't quite feel right no matter how I juggle components, which leads me to believe that there is weird stuff going on sometimes. So even at the best of times just purchasing a frame/fork can be a bit of a crap shoot and on-line even more of a risk.
I don't know how many times I have seen frames quoted as one size or other on-line, and have the pictures appear to tell another story entirely. "I don't know much about bikes, but it's a 25" frame for sure, I measured the wheels and they're 25", the tires are glued on, but one's coming off and they're flat, I think you'll have to replace those." ;-)
Marcus Coles
London, Ontario, Canada.