Order the cheap $6.99-9.99 digital calipers from Harbor Frieght, they are accurate enough to measure tire size.


http://www.harborfreight.com/catalog...ult?q=calipers

In general you'd want the fender to be 8/10mm wider than your tire. So a 25mm tire would need a 32-35mm fender. The 700c fender will fit 700c & 27" wheels, then pick your width 35mm or 45mm etc; 650b fenders will fit 650's and 26" MTB wheels.