Here's a trick I learned from making all those Blooming 9 patches -
Don't ever cut the plain/solid squares until all the pieced squares are sewn and squared up.
And then, whatever that measurement happens to be, that's the size you cut the plain squares.
Yes, in a perfect world, even if you weren't doing such a free-form design, every block sewn would be finish to the same exact size.
Well, the world ain't perfect.
So you can stress over the fact that yours aren't - which is in total opposition to the whole free-form movement - or you make sure they're the same size (by squaring them up), and now the blocks will go together so smoothly you'll be done before you know it.