Seatpost Diameter: 25.4, 26.8, 27.2
On a frame, this is the inner diameter of the seat post tube. This must match the outer diameter of the seat post.
The seat post diameter is usually stamped near the bottom of the seatpost. If not, you have to measure it. On my Rocky Mountain, the new alloy post I bought is stamped 26.8 mm, but I had to measure the old post. Sheldon Brown says 27.2 mm has become more of a standard. However when you measure a seat post with a caliper, you discover they can vary slightly, and are not really exactly round and uniform. I initially measured mine to be 26.8 mm, and in places is not completely round.
At MEC.ca website, you can see a typical seat post called MEC 350mm Alloy Seat Post. 350mm is the length. That model comes with the following diameters to fit different tube sizes: 25.4, 26.8, 27.0, 27.2, 30.0, 30.9