- Ah oh, I did this same thing but only made 2 solid blocks, uff thank goodness...I realized wow these won't be the same size, luckily it was just a 'for fun' side project. I will keep an eye on this thread to see what you end up doing so I will know now what to do with all my pinwheels I've already made...
-Best of luck to you :)

This circumstance is exactly why I always piece blocks first before I cut plain alternate blocks.
It also helps to piece the more complicated blocks before the lesser pieced blocks for the same reason.

Jan in VA

Experience is an effective teacher!

In the meantime, though - - -

Do you have any template plastic? If so, I would cut a 5 inch square of it and draw diagonal lines on it.
Then I would use that to square up my pinwheel blocks. Or buy a five-inch square ruler that has the diagonals on it.

I would square up the pinwheel blocks to be the same size as the solid squares - if they are different sizes, you will have a mell of a hess trying to put the them together.
It definitely helps to have the intersection marked.

I did that same thing, first time I made pinwheels. It didn't take me long to do the pieced blocks first, then measure it and cut solid alternate blocks. This won't help you now but will if you make another like it.

You want to line up the 2.5" lines (BLUE SOLID LINE) with the upper right quadrant of the pinwheel block, as well as the diagonal (BLUE DOTTED LINE).
Make sure that you have enough "block" left on the other side of the RED LINE (5" border).

[ATTACH=CONFIG]376977[/ATTACH]

If all is okay, trim the right and top edges of the block without moving the ruler or the block.

Turn the block so that the two trimmed sides are now at the bottom and left on the mat.

Place the ruler so they line up with the 5" lines (RED) at the bottom and left, with the untrimmed edges at the right and top.

Line up the center and diagonal again in the upper right hand corner.
Trim the right and upper edges.

And you should have a perfectly sized Pinwheel block.

In theory anyway. :D

Oh, one last note.
If you're using a ruler that has the added 1/2" measurement, like the one in the picture (GREEN circle), ALWAYS make sure it is on the bottom left, away from where you're measuring and cutting.