I have Mavic CXP 22 rims. I have to enter the wheel size in my Sigma computer (resetting after battery change). What is the size of the wheel. Currently it shows 2.155. Anyone know?

700x25... ~ 2105

2155 is 700 x 32...

Do a rollout and enter that.

A "rollout" is absolutely the best. I have three bikes. When I started doing the "rollout" measurement, all three bikes suddenly matched each other and gps down to the 10th of a mile for my commute routes. Everytime I mount new tires (or swap my studded snow tires in and out) I do it.

It is described in section 12 of the Sigma bike computer manual you can find here:

https://www.sigmasport.com/en/images/pdf/Manual 20PURE 201_Montagebogen.pdf

The actual rolling circumference of a tire is dependent on inflation pressure, load, sidewall flexibility, sidewall height versus rim width...anything that deforms a tire. AND...there are no tire size police. So the most accurate way to enter that wheel size factor is to roll it. For almost every bike computers it is the circumference in milimeters.

I use a sidewalk and line up my valve stem at the bottom, centered on a sidewalk "seam". I then roll my bike forward until the valve stem is centered "down" again. I take chalk or make a scratch on the sodewalk with a stone. Then I measure with a tape measure. If I'm feeling a little OCD I do it three times and take the average.

If you used 2105 but the tire was actually 2115, instead of thinking you're going 15 mph on average, you'd really be going ~15.07126 mph which means that after a century, instead of 6.67 hours you'll be done nearly 4 minutes sooner...

A rollout is best. Also, Sigma computers are German and providing the specs down to a single mm is perfect

edit, that's 2,155mm