Your block is square. The triangles give the illusion of making it appear rectangular.
So, for a 12" block you would cut the squares 4.5". Cut the Half Square Triangle blocks at 5" and trim down. Each 5" square should give you 2 Half Square Triangle blocks. You'll need 2 pairs (4 total) for each Spool. Draw a diagonal line on the back of each "light" 5" square. Place each light with a dark 5" square right sides together. Sew a scant 1/4" away on both sides of the line you drew. Cut on the line & gently press out the blocks. Trim to 4.5".