For a mudguard to work correctly, it needs to sit close enough to the tyre that water is captured, without being too close that it will consistently rub. Beyond keeping an eye on what the maximum tyre clearance of your fender of choice is, keep in mind that if a rock or something similar gets jammed between the tyre and the fender it may cause the wheel to come to an abrupt stop — a big problem if it's your front wheel.
Some fenders have safety release clips where the fender can pop out of the way and allow the tyre to keep spinning if something does get wedged.