The size of your tire should be written on the sidewall. The fender should be larger than the tire, but small enough to fit your frame. I personally prefer Planet Bike Hardcore fenders, which are a good value and relatively easy to mount. If you have 700c wheels, you'll probably want the hybrid size, or the ATB fender for 26" wheels. If you buy them from the same place you bought your bike, they might be willing to mount them for free.