Can anyone tell me the seatpost diameter of a 2004 Cannondale R1000. I picked up a Thomson seatpost but it seems to fit much tighter. It is a 27.2 and is 330mm long. Also is there a difference between the Mountain bike post and the road?

If a 27.2 fits tightly, then that is the correct size. If you have too much post in the frame, it can cause some binding, since the seat tube in probably not perfectly straight. Compare the minimum insertion length (usually around 7cm) to the amount of post in your frame. You may be able to cut off quite a bit. I had to do that with a Thomson in my LOOK frame. I cut off about 9cm.