If the large square is 5" finished and sashing is 1" finished, then the small squares are 2" finished or 2.5" unfinished. I would make 3 units for this quilt . The 5" square, 2 small squares, and 3 small squares. The 2 small squares get sewn to the large square with sashing between. Then you can lay them out and see where you need to add sashing strips. Then sew it together in rows.