Up until the early '80's, this was a fairly easy question to answer. You would stand over the frame of a bike, and if there was an inch or two between the top of the top tube and your tender parts, that was the right size. Bikes commonly came in frame sizes two inches apart, so there was not much question whether the 21" or the 23" was the "right" size.
At that time, in the world of mass-produced bikes, the difference between different size bicycles was that the larger sizes had longer seat tubes and head tubes , so the top tube was higher. This was usually the only difference between frame sizes.
In a given model, the height of the top tube would vary, but the length of the top tube and every other part of the frame would be same, whether the bike was a 19" or a 25". A person who buys a 25" bike is likely to have a longer upper body than someone who buys a 19", so the larger rider will likely feel cramped by having the same length top tube that puts the handlebars too far away from the 19" rider. The only concession to this difference was that the better builders would supply a stem with a longer reach on larger frames, and a shorter one on smaller frames.