Seat post length is dictated by leg length, leg extension (straight or partial bend) and foot extension (flat or in my case pointing slightly down). I have a Brompton M6R with extended seat post (it came with the regular but they swapped at no cost a week after purchase). I need to flip the seat mounting bracket that it came with because it is still not long enough. And I stand about 5' 9". A coworker also has has a Brompton with the extended seat post, is happy with the seat height and stands 6' 7". So you really need to try out the bikes and if not sure, rent one for a weekend if possible.

Iain