When you sew your squares together and add your border before cutting, you are correct, you are going to lose a lot. Your finished squares you are cutting will end up 7 1/2" (finished size). I would say to do 13 x 13 and then add your borders before you cut. You will end up with a quilt that is roughly 105" square. Be sure you add the border before you start cutting, otherwise you won't end up with pinwheels on the edges.