thats a great idea.
this is the largest circle cutting that i know of - it goes to 30 inches.
http://www.phillipsfiberart.com/stor...d&productId=23
i have this one that goes to 10 inches but i can get a 20 inch circle from it. you're supposed to use folded fabric but instead i just cut it on the whole piece and flip the template over and cut the other side getting 20 inches total.
http://www.ezquilt.com/products/acry...ycirclecut.htm