To square a 6" finished block, you want it to square to 6.5". The center block will be 2" square. The rest will be 2.25" square until you stitch to another block.
If you tried to do that & are still off, only way to get it perfect is to unsew. I have that problem right now with my 9 patch. I mis-cut and made the inner strip 1/8" too small. So I'm just changing my finished block size from 10" to 9.75". I split the 1/8" difference across the piece so my blocks are only 1/16" off on either side which a 3-year old (or most non-quilter adults) will never notice. If you're not entering it in a show, chalk it up as a good lesson, square your blocks smaller and move on.
If I plan to make a bunch, I will buy that size of a square ruler at Joann when they have a 50% off coupon (they have one through today). You may also need to block it & to do that you just lightly spritz with water & tack to carpet with T-shaped pins.
Here's a link:
http://youtu.be/cdUTLkNc0po