What counts in sizing and fitting a bike are the points where the bike intersects with the body. The relationships between the saddle contact point, pedals, and the handlebar grip area, determine the riding position. These relationships can be affected by a number of factors including frame size, saddle adjustment, crank length, stem size and adjustment, and handlebar design.
I’ve learned through many years of riding that, for me, the most important factor in bike fit is handlebar height in relation to saddle height (after setting saddle height, of course). I prefer the primary grip area of the handlebar to be 1cm-2cm above the top of the saddle, when the saddle is adjusted to the proper height (more on saddle adjustment here). Too low and there’s too much weight on my arms, too high and there’s too much weight on the saddle. This is a personal preference based upon how I ride, and others will certainly come to different conclusions based upon their physique and riding style.
What I find less important is the length of the cockpit, or in other words, the horizontal distance from the saddle to the handlebar. If the saddle to handlebar height relationship is correct, I feel equally comfortable on bikes of different lengths (within a reasonable range). Consider the photo above. The Surly LHT is a 56cm frame and the Rivendell Sam Hillborne is a 60cm frame. As you can see, the Rivendell is longer, but because the bars are at my favored 1cm-2cm above the saddle, I still find it comfortable. In fact, these two bikes both fit me quite well for how I use them. The Surly is a city bike for hauling weight in traffic, and its shorter front is good for that type of riding. The Rivendell, on the other hand, has a more open cockpit and the stretched out position is better for longer rides on open roads.
Where it gets tricky is when you start looking at different handlebars, stems, and top tube angles. When I purchased my Rivendell, I knew from the start that I wanted to spec the bike with Moustache handlebars. These bars have much less rise (they actually drop a bit) than the North Road bars on the Surly. To get the grip area at 1cm-2cm above the saddle, I knew I had to either use a stem with an extreme amount of rise or choose a larger frame to bring the front end of the bike higher. Since I didn’t want an exorbitant amount of stem showing above the headset, and I also wanted a more open cockpit than on the city bike, I went with the larger frame. On the other hand, if I had planned on speccing Albatross or North Road bars, I would have gone with a smaller frame size because of the handlebar’s greater rise.
Some modern sizing methods use upper body measurements and top tube length to determine frame size. While this may work for some people (but usually only if they’re speccing drop bars), it invariably places me on a bike that is too small. One of my bikes is a 54cm and it was sized using the top tube method. I’ve never been able to get the handlebars at my preferred height in relation to the saddle on that bike. The bars are too low and the cockpit is cramped, even though according to a popular “scientific” sizing method, this is the correct frame size.
All of this is a long-winded way of saying bike sizing is complex and I don’t believe it can be boiled down to a codified method. The photo above demonstrates that with careful component choices it’s possible to achieve similar riding positions on bikes of widely differing sizes. The number of variables in the process, including the intended use of the bike, frame geometry, saddle set-back, and handlebar/stem choice, make the process more an art than a science. Seeking out a knowledgeable salesperson or builder who has experience with transportation bikes, clearly understanding and stating your personal preferences, and remaining flexible in your assumptions, is much more likely to land you a properly sized bike than any mathematical formula.