I've done D9P and quilts that look like D9P (sashed with cornerstone) with 5" and 6 1/2 inch squares. 6 1/2 worked just as well as the 5". I used it on what I call "toddler" quilts, which is big enough for belly time on the floor or small enough for a "dragger" for those kids who love to drag around their quilts. (somewhere around 50X50)
bkay