Fenders need to be 10-12 mm wider than the tires. So if your tires are, for example, 700x32c then the fenders should be 42-45mm. I have 700x32c tires and have a 45 mm fender on it with plenty of clearance. The reason that the fenders should be larger is in the event your tire picks up a blob of mud, a rock or whatever it won't get caught between the tire and fender (hopefully). SKS fenders of a secure-release built into the front in case something does get caught up there.
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