I have a Dahon Vitesse D7.. Have lost the seat post. Can you let me know what is the size of the seatpost will fit to the mentioned model
Go to a bike shop and get it measured.
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.