Hello and Welcome to the board :D:D:D
Accurate cutting and sewing accurate 1/4" seams are necessary to get the seams to all line up :D It does take some practice.
If you have accurately cut the squares at 9" and sew two of them together with a 1/4" seam, this piece should now measure 9" X 17 1/2"
If it is smaller, your seam is to large, if it is bigger, your seam is to small.