cut your squares 7 inches square.... then use a tutorial like this
http://www.psiquilt.com/2010/02/half...-tutorial.html to make your half sqaure triangles that make up the pinwheel!
the math for making HST like this is by adding 7/8 inch to the unfinished size of the HST. so if the unfinished size is 6 1/8+7/8=7 inches!