Join Date: Apr 2006
Location: Gainesville/Tampa, FL
Bikes: Trek 1000, two mtbs and working on a fixie for commuting.
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If you pull it out, it should have it stamped on the post. Flip off the quick release and pull up from the shaft, not the saddle (I've heard of problems with the suspension seatpost coming undone when pulling up by the saddle), and look for any numbers on there - it should have a "min insert" or "max height" and then a number like 30.9 or 31.6 or 27.2 and that would be the size.
Chances are it is 31.6mm, but I would check before committing to my answer.