There's a size chart on page 15 of the manual that has great instructions for measuring and entering the wheel size.
You can get the manual from Sigma's website at this link
Method "B" is the most accurate way -- make a chalk mark on the tire and stand the bike up next to a wall with the chalk mark at the bottom.
Have a friend make a chalk mark on the ground at that point.
WITH YOUR FULL WEIGHT ON THE BIKE, move forward for one rotation of the wheel -- until the chalk mark is on the bottom again. Do this as straight-line as you can.
Have your friend make another mark on the ground at this point, and measure the line between the two points on the ground.
This is the wheel circumference, and if you do this with normal tire inflation and with your full weight on the bike, it's the most accurate way to calibrate your cyclocomputer.
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