Originally Posted by elemental

The key to modern sizing is top tube length (unless you are either terrified about standover height or have such strange proportions that it's a big issue). All you need to do is figure out what top tube length you fit, and figure out what size that corresponds to. Modern sizes usually come from the now-archaic tradition of sizing a bike by its seattube, but even this is problematic because they can be measured center to center or center to top, and don't take into account bottom bracket drop.

So all you need to do is figure out your top tube size, look it up here, and find your listed size. These do seem to run a little big- I usually ride a 56ish (ideally around 555mm) top tube, and this would have me in either a 53 or 55. The top tubes are much longer than the listed sizes (seat tubes) in the smaller sizes, but it seems to even out as the sizes get bigger.