Originally Posted by wkg

You can use a measuring caliper.

Either that or most well outfitted LBS's have a set of cylinders that increase in width along the length of the cylinder with labeling that tells the width. So you drop it into the seat tube and where it stops is the width of the seat tube. These are good because you need to be within a tenth of a millimeter and sometimes it's hard to measure perfectly across the diameter with a caliper.

Edit: for size of the clamp, however you may need to measure the outside diameter of the seat tube, for this you would use a caliper measuring device. Your LBS will know for sure though so probably go talk to them. Also they should be able to order you the clamp once they get the size right and that would save you on shipping.