OJ Simpson Glove?
There are a couple of different ways to measure the bikes, but for a "vintage" bike with a horizontal top tube, I'd be looking for something on the order of 58 to 60cm. Maybe down to 56cm if you like a fairly aggressive stance, with a fair amount of bar drop.
I like to measure the size by measuring center of bottom bracket to the top of the top tube, measuring along the seat tube.
Also, a horizontal line from the center of the seat tube or seat post to the center of the head tube, and it should be similar.
Nonetheless, fit is personal, and up to you. But, if you immediately turn around and buy a longer seat post and a foot tall stem to "make it work"... then it probably doesn't fit.