When I look at fenders online, I see a lot of them that specify that they are designed for a certain width of 700c tires.

How would I buy for a 27" wheeled bike? Would be safe to assume that I should buy based on 1 & 1/4 (31.75) + 4mm so I should buy a fender able to handle a 36 MM width 700c tire?

Or am I missing something? Do I need a 27" specific fender?

Also, does anyone know by experience if a 1989 Cannondale touring bike would handle 1 & 3/8 " tires on 1 & 1/4" rims with or without fenders? Would there be any realistic reason to consider 1 & 3/8" instead of 1 & 1/4" (I am a clydesdale)?