The diameter is spec'd by the wheel size, ie. 700c, 26", etc. But the width is the actual width, so a 35mm fender will be 35mm wide.

You want a fender wider than the tire, but you don't need it much wider because the vast bulk of water thrown comes off the center zone of the tire. For mud, you might want a more generous overhang.

One thing they don't specify is how much radial clearance there is. But fenders are flexible, and that will be mostly determined by the height of the fork crown or seat stay bridge, with you adjusting the braces to maintain that all the way around.