Each 9 patch has to be the same size. Mine all have to be squared to 8" (7 1/2 inch finished). I just put them on my cutting matt and cut to that size. since my squares are finished at 2 1/2 inches I try to line up the center one so the end ones will be 2 3/4 because of the 1/4 inch seam. If you don't square them up to the same size, once they are sewn together they will pucker some if they are off.
They lay so much better when they are squared off. It makes a huge difference.
Barb