I have some SKS P35 fenders with 28mm tires, and SKS P45 with the same tires.

While you can fit a 28 under a 35mm fender (3.5mm clearance), the issue I've encountered is side-spray. There's not enough overlap to really keep all the spray from getting back to you, even if you put on a mudflap.
I've noticed similar issues with my 35mm studs under the 45mm fenders. Sure, they fit with lots of clearance, but the tires sling a lot of grit and the fenders don't contain it all.

A 5mm clearance on a plastic fender could cause some issues at the metal brackets where the struts attach. The plastic fender is 45mm wide for clearance, but that bracket is riveted inside the fender and sucks up 1 - 1.5mm of total clearance. The 2.5mm clearance you'd hope for with a 5mm difference between fender and tire size is reduced to 0.5 to 1.0mm at the bracket, meaning you have to set the fender farther away from the tire, which significantly reduces the ability to effectively block side-spray.

Since you asked about a 26" fender; it probably won't fit over a 700c/29er wheel. The curve will just be wrong, like trying to put on a size Small helmet if you wear an Extra Large.
With the larger fenders, what you need to measure is the width clearance at your fork and your brake bridge. A fender like the 65mm 29er model might be too wide to clear your frame without modification. I had to modify a pair of SKS P65 fenders to clear the fork crown on my utility cruiser.