Currently I'm on a pair of Vittoria Corsa EVO CX II 23mm. Ever since I've been on 23 mm but now it's getting a little harder to find. Many dealers seems to be out of stock. How are the differences to the 21mm regarding rolling resistance and riding comfort?
I rode both. Used to ride the 21s before the 23s came out. Rolling resistance is not much different I think. The 23s do ride smoother though. A bit more meat on the tire just feels nice to ride. Obviously it depends on the tire pressure and surface you ride on as well. You could run the 21s with a bit less pressure.