Road bikes vs mountain bikes
Sizing is a really important aspect of choosing a bike that will provide a good cycling experience. A poorly sized bike can cause health problems and can make cycling a unpleasant drag. There does not seem to be a universal standard for bike sizing due to the variety of designs and geometry. Size varies between manufacturers and intended use; touring, racing, commuting are all slightly different.
Developing a good relationship with your local bike shop is a good opportunity to try out different sizes and frames and get measured up for the right one to suit you. This is especially useful if you have non standard body geometry. You can also adjust the fit to your specifications, racing, endurance, comfort etc.
Most brands will have a guideline to get you in a range. However, one brand may place you solidly on a 58cm frame, but a different brand might place you between a 58 and a 60. You really have to test ride several sizes and brands to see what fits. A proper fitting will take into account your geometry and the bike geometry and try to get a match. It may involve swapping stems, adding headset spacers adjusting saddle setback. These adjustments make the frame feel bigger or smaller to get you comfortable.
The old traditional scheme for sizing a road frame was "standover height". You'd stand with your feet flat on the ground, straddling the bike. If you had "comfortable" (for a male) clearance of the (traditionally-positioned) top tube then the bike was the right size. This is by no means a steadfast rule but it is a good starting point and bikes are no longer the traditional diamond frames. There are other aspects of fit, such as "reach", that usually follow overall fit, but not always.