I got an Airnimal Chameleon (folder) frameset last year. They come in Medium (52cm) or Large (56cm), which relates to the equivalent top-tube length. Seat height is of course determined by how much the seat stem is extended.

For me, comfort and handling has a lot to do with effective top tube length, let's call this the lounge. I like to be able to relax in the lounge. When I stop, though, it's the ballroom that becomes most important ;O)