Measuring for Fenders
The basic rule in fender fitting is that the fender should be at least 8mm wider than the tire; 10-12 mm is better. So a 35mm fender will fit up to a 25-26mm tire; a 43mm will cover a 32-35mm; 45 covers 38mm, etc. The reason for this clearance is that otherwise little pebbles, sticks, and other debris that are thrown up by your tire will get lodged between the fender and tire. Of course many tires are smaller than their advertised size, so it's wise to actually measure the inflated tire.
A fender's width is measured on the outside, not inside. So to determine if a fender will fit you'll check clearance at the brakes, in the fork crown, and in between the seat stays.