This circle-cutting router jig is fully adjustable and will cut a circle with a minimum diameter of ~10″ depending on your router size and up to infinity depending on how long you want to make your swing arm. It is fully adjustable within all the fractions 1/64″, 1/32″, 1/16″, 1/8″, 1/4″, and so on up to the full inch and everything in between.
All you need to build it is some scrap plywood and a few bits of hardware.
Watch it be built here.