I think the most important dimension to know is standover height. I have of late tended to buy 55-56 cm frames, which I may just be able to straddle flatfooted. Some will tell you that you should straddle the frame with 1" of room, but I find that unnecessary, and the larger frame allows raising the bars without too long a stem quill. You can sort of equate the seattube length to standover height assuming typical BB height, but there are exceptions. Some Mercians and some Woodrups, just to name two, have very high BB's. So it is better to know standover height and decide what standover you are comfortable with. The toptube length can almost always be compensated with stem extension, as classic stems commonly came as short as 60mm, sometimes even shorter.
Of course, compensating for frame dimensions with seatpost exposure, stem quill exposure and stem extension, and saddle fore/aft adjustment doesn't theoretically position you ideally, but I can almost always set up a bike comfortably as long as the standover height is reasonable, which for me usually occurs with a 52-56 cm seattube center-to-center.
Now if you only intend to have one or two classic bikes, perhaps you can hold out for the perfect frame dimensions, but if you collect a larger number of used classic bikes, or even new stock frames, it is just too difficult to find them with all the ideal dimansions.
Custom frames, of course, are a completely different story. If you are going to spend the money and time on a custom order, you should have all the dimensions exactly to your liking. On my custom-built Caygill tourer, for example, I ordered (I think) a 53 cm seattube, for a bit more standover room, but also an extended headtube to allow higher bars. For the realtive few custom frames I have purchased, I tend to favor "square" geometries, i.e the toptube the same length as the seattube. In my case, I'll usually order a custom frame in 53cm square or 54cm square size.
Big Spring, Texas, USA
> From: Robert Goughary <email@example.com>
\r?\n> Subject: [CR] vintage frame size question
\r?\n> To: firstname.lastname@example.org
\r?\n> Date: Friday, March 20, 2009, 12:33 PM
\r?\n> This is a good one for all you with more experience than me
\r?\n> - particularly those of you that have built frames or sized
\r?\n> people up in shops, etc...
\r?\n> when buying a vintage frame sight-unseen, how do you know
\r?\n> if the frame will fit? Not - what would be the perfect fit -
\r?\n> but let's say you run across a Richard Sachs from the
\r?\n> seventies and the only thing you know is seatpost height -
\r?\n> what do you do?
\r?\n> I am 5'11" or so - normal build and size - how do
\r?\n> i know what are the fitting extremes? - what is to small,
\r?\n> way too small and too big and way too big? Example - can a
\r?\n> 6' guy fit on a 54" frame? Or a 60cm? (seatpost
\r?\n> I have a couple bikes that fit - so i'm more concerned
\r?\n> about the random find online or maybe at a flea market,
\r?\n> Rob Goughary
\r?\n> Stamford, CT USA