If you're looking to get an older road frame and do a conversion, note that sizing is a bit different - the old adage was "fistful of seatpost showing" - people tended to size larger than they do in today's world of compact geometry, etc.

Sheldon Brown has a well thought out article on sizing on his site. I think his method works well in 99% of cases. In short: rely on top tube length first, but use a inseam length as a reality check.

(If you're getting a frame with true track geometry, then ignore all of this.)