Find your smallest block and square them all up to that size. It doesn't matter if the pattern said that they are all to be 12 1/2 and your's come out (once squared up) 11 1/2 it will just make the end result a little smaller. As long as they're square it will be okay.
p.s. I think pinwheels or any other block with diagonal seams is harder to get to come out right than if you're using square pieces.