You have to sew a 1.5" strip to the side of the center square which makes it a rectangular unit. Then you add 1.5" to that side and 2.5" to the other side. If you do the math (minus seam allowance) 2+1+1+2 = 6 which would make it 6.5 with seam allowance. I suspect you didn't sew that first 1.5" strip to the side of the center square.