This is definitely not a stupid question. As people have explained, the stated width is the outside width of the fender. Take off a couple millimeters for the inside width, and allow a few millimeters on either side for tire clearance, to get an idea of what size tire the fenders can fit over. Its definitely a good idea to measure the actual width of the tires on your bikes. In addition, you should also be sure to read over the Velo Orange instructions for fender installation. Their instructions are quite good and will also direct you to the areas of your bike that you'll need to measure for fender fit (under fork crown, between brake calipers, between rear tire and seat tube, between seat stays, etc.).

I have Velo Orange fenders on two of my bikes and I'm very happy with the quality. The first time I installed them was a learning experience, but the 2nd time went much more smoothly. Good luck!