If the block is coming out too small, there are a couple of things that might be happening:
1. you are not cutting accurately in the first place. Did you know that the ruler you use can make a difference?
http://www.quiltingboard.com/main-f1...l-t142312.html
2. your seam allowance is too big. If you need help with achieving an accurate 1/4" seam allowance, I have a post here that might help:
http://www.quiltingboard.com/tutoria...ce-t89997.html
Andi