Correct - it depends on what fenders you choose. Planet Bike Hardcore Hybrid fenders do fine on my 35s, and barely fit my 40s. They are 43mm wide, internal width, but the hardware fits on the outside, which is a very good thing, Serfas fenders, and Bontragger fenders have hardware that intrudes on the inside of the fender, taking up valuable room. A set of Serfas, at 42mm wide, just barely accomodate a set of 37s, with no room to spare. A set of Bontragger 43mm wide, just barely accomodate a set of 35s.

Attachment hardware makes a HUGE difference in "effective" width.