I saw this question and answer on what is the right seat post to install on a frame and thought it made sense. From Classicrendezvous@bikelist.org
"How does one determine the proper size seat post for a frame that comes without one. I've measured the inside of the seat tube with no tension on the binder. I understand that the post has to be smaller in diameter than that measurement but how much smaller?
They should be very close. Go with the next size down from what you measure. i.e. if the post measures as 27.32, go with a 27.2. Keep in mind that the very top of the seat tube often gets crimped in a bit, and will actually measure smaller than if you could measure the tube an inch lower, so it can be a bit tricky.
"Beer can works great for a shim."