If you don't have a caliper, you can employ basic geometry: wrap a strip of paper tightly around the seatpost, use a sharp knife to mark where it overlaps. Lay flat, measure accurately in mms, divide by pi (3.14) and that's your diameter. A caliper is better, but this will get you close in a pinch. Do it a couple times and average the results.

Or you can cheat and pull the old seat post out. It may have the size inscribed on it.