I like my vintage bikes to fit as they would have been fitted when new. I agree with Monkeylad that this means the top tube length is more or less constant, but seat tube length can vary a lot.
In the 1980s, I was put on a 57 cm c-t racing frame. I ride a 61 c-t 1950s racer, a 62 c-c 1950s randonneur and a few other bikes. All have top tubes that are on the order of 57 c-c. In some cases, it is hard to find bikes that big - few French and Italians were 6 ft tall back then.
That said, the 57 cm bikes from the 1980s now feel distinctly too small for me. Riding more long distances and being less fast than I used to be, I find that higher bars are better.
Consider the following: Racers used to have relaxed bikes and relatively high bars when stages were long and roads were so-so. With the current short stages, advances in doping and the resulting increases in speed, bars can go down, because racers pedal so hard they don't have any weight on the bars.
But our riding in the U.S. is much more akin to Europe in the 1950s: Our centuries involve many hours in the saddle, and our roads certainly are bad and getting worse. So fitting a bike like they did for Coppi might make sense for many current American cyclists.
Interesting to see the previous thread about Coppi riding 59 c-t bikes. I bet he'd be put on a 56 or smaller (c-t) these days. How tall was Coppi, anyhow? I know he had very long legs, but just wondering.
Jan Heine, Seattle