Here is a complete article on how to make your own adjustable circle-cutting jig for a hand held router.
Unlike many a circle-cutting jig for the router, this one overcomes a common limitation -- cutting a circle with a radius shorter than the distance between the router bit and the pivot on which is spins.
John English came up with the ingenious jig you see in the photo above. It doesn't rely on a pivot point, which means it can be used to cut range of circle sizes.
Here is the article on How to Make a Circle Cutting Jig for a Router