I agree with Anniedeb. I think the visual impact of this quilt will be lost if the circles fall way off the top of the bed. Have you considered adding borders instead to make it larger?
The quilt is actually constructed of 36" log cabin blocks with a 2" sashing. You haven't stated how big you want the quilt to be. Adding half blocks might make it too big (18+2+36+2+36+2+18=114), but you could add partial blocks all around.
The other problem with making wider strips is that you'll have to figure out the new dimensions for where each strip changes color in order to form the circle.