The standard Brompton seatpost is 520mm. As far as I know, it has always been like that.

But the actual seat height depend also of the saddle height and pedal thickness (+ shoes sole thickness + pant thickness).

The standard seatpost typically fits for people till 1m75-1m80 )depending of other factors including cyclist Xseam, women have typically longer legs and shorter torso than men).

There are alternative titanium seatposts in 520, 550, 580 and 600mm.

Carbon is a bad solution on a folder as it is easily scratched by dirt when folding the bike. Also carbon doesn't like many decompression/compression cycles that occur when folding/unfolding a folder.