bobo, back in the day when an 80's bike was modern, a rule of thumb was about 10 inches off your inside leg measurement. If you are of an average build for 6ft, then I'd say you are either a 22.5inch or 23.5" this is measured centre of bottom bracket to the top of the seat tube, known as centre to top.
This assumes that the seat tube doesn't extend past the top tube. If the seat tube does extend past the top tube, you would quote a Centre to Centre measurement.
You are best having a sit or a ride on a retro bike before you spash any of the folding stuff.
send me a pm with where you live in Exeter as I have a friend who has both these sizes and would probably be happy for you the try them.