I'm assuming that when you say it's "too big" the piece that is larger is the rectangle with the white square vs. your 9 patch + first rectangle. If this is correct, my guess is that your seam allowance is off. You should measure your 9 patch before you attach the other pieces. It should measure exactly 5" X 5". If it doesn't, it means that either you cut it wrong (which you've checked), sewed it wrong (see below) or when you pressed it, you ironed a tiny crease in one of the seams.
We talk about a 1/4" seam, but you don't measure the seam, you measure the resultant patch. Here's the standard way to check:
http://www.quiltingboard.com/tutoria...ce-t89997.html
Here's a link to your pattern & it looks like the cutting directions are all correct.
http://www.mccallsquilting.com/mccal..._Block_Pattern