With the size of a flywheel you are restricted by the diameter at which the starter motor is. If you have got the 18RG flywheel out at the moment, and the flywheel face is built up larger than the ring gear its fair to say you wont get a bigger flywheel in and therefore a bigger clutch, as GT said the clutch bolt holes are usually as far out on the diameter of the flywheel as possible, the cover sits very close to the edge of the flywheel.
I don't think im telling you anything you dont know here haha