Sheldonbrown, sheldonbrown, sheldonbrown!!!!!!!!!!!
Search Function, Search function, Search!!!!!!!!!!
Ha ha, i just never get tired of saying these. But as i am a bit of a Twenty "connoisseur" and want to help you out a bit, i will just qoute some of the stuff of:
"Twentys are designed to take a 1 1/8" seat post, and come with a heavy steel one, which may not be long enough for taller riders. Since I'm long-legged, I used to have to extend the stock seatpost by attaching a smaller seatpost to the top of it, but now, thanks to mountain bikes, and "oversized" designs, suitable modern aluminum seatposts are available from stock for a reasonable price, in the 28.6 mm diameter"
28,6 is a rather common size so guess you should have no problems with your requirements.
Additonally there is a plethora of info on Twenties (how to acquire them cheap, upgrading etc etc.) on these boards, other people's pages and my page (see sig.) really suggest you check those out too...