Start choosing mudguards by checking the bicycle itself. Note the wheel size, tyre width, mounting points and space inside the fork. A luggage rack and the brake design can also limit the choice. Matching the wheel diameter alone does not establish compatibility.
A useful width guideline is a mudguard about 10 mm wider than the tyre, leaving roughly 5 mm on either side. For a 40 mm tyre, that means initially looking at a guard around 50 mm wide. Allow space above the tyre too: the general selection guidance recommends about 15 mm. Final suitability depends on the particular model’s fitting requirements and the available room.
An online selection tool can narrow the options, but cannot always establish mounting details or conflicts with other components remotely. If the fit is unclear, take the bicycle to the shop. Seeing it in person allows dimensions and mounting solutions to be checked together.