What's difference between 1/2 and 9/16 inch pedals?
The measurement is for the diameter of the pedal spindles, where they thread into the cranks. You need to find out whether your existing pedals' axle is 9/16" or 1/2". If there is no measurement indication on your existing pedals or the paperwork which came with your bike, ask your LBS if they could test out with some existing pedals to find out what size you need.