Yup, that’s the size. The bike trade, unfortunately, keeps its customers confused by describing tyres in a great many different ways, so I made a page to explain it: search for ‘tyre sizes’ at cyclinguk.org.
Mudguards are often sold without any tyre size guidance, just the width of the guard. In that case, you generally want a guard about 10mm wider than the tyre section. Too wide and the guard adds unnecessary wind resistance, too narrow and it’s hard to keep it from rubbing one side or other of the tyre, especially on rough roads where the guard tends to flap about. So a 35mm guard is good for tyres up to 25mm, or 28mm at a pinch – literally! The product you’re looking at is most likely a 45mm guard.
If a mudguard is too close to a tyre (especially one with a prominent tread) it can catch onto it, get carried around by the wheel, and jam into the frame. So it’s important to have enough clearance. And stay-release fittings are required for safety on a front guard, to let it break free if and when that starts to happen, because a front wheel jam invariably sends the rider over the handlebars!
This was first published in the February / March 2014 edition of Cycling UK's Cycle magazine.