What kind of fabric is it? If it is a stretchy type you can use an iron on interfacing as a stabilsier, that's how t-shirt material is used for quilting.
The more you unpick and resew, the more distorted the fabric gets.
Be careful when you're ironing too, just "press" the iron gently down the seam, don't wiggle it around because that distorts the fabric as well. I discovered that watching a quilting show on tv and it's made a HUGE difference to my quilting! lol
Once the sashing is on, you can trim the blocks all to the same size. But wait until you have all of them made first, and trim them to the smallest size.
:-)