Does anybody know these measurements? I checked Thor's site, and he says that they fit all 20" bikes.

Anyone with a Speed TR would know ... but it's cheaper to do this:
http://www.bikeforums.net/showthread.php?t=190142
$88 and a hacksaw.
(BTW, the slit in the stock seatpost now has a stress relief hole at the distal end of the slit)

Sorry, don't know the measurements. I have a Thud Buster on my mountain bike with my treasured Brooks saddle and wanted to use it on my Speed P-8. I bought the two piece seatpost from thorusa. The top section is 27.2 mm and a 27.2 or smaller w/ shim Thud will fit. You easily can swap the Thud between bikes while the Dahon specific Thud w/ its huge dia. post is more limited usewise.