Require seatpost size of Dahon Vitesse D7
Go to a bike shop and get it measured. 

They'll have some seatpost measuring tool like this:


You basically stick it into the seat tube and it tells you what size seatpost you need.

Alternatively, you could use a vernier caliper to measure the seat tube diameter on the inside (this is key!). 

Note that the seat tube diameter is only one dimension; you also need to know what length tube you need. Most seatposts have the same clamp type as well, but you may want to get one with extra fore/aft adjustment or whatever. So, going to a shop and buying one isn't a bad idea if you don't know what you need.