This might not be what you want, but could help. If you know the largest block that you want to make, you could go ahead and make that block. Then you could add simple fabric strips to the smaller blocks to make the smaller blocks the same size as the larger one using the strips around them. You could use a contrast fabric to make the smaller blocks stand out or you could use a background fabric that matches the larger blocks and the smaller blocks will just show up in different places around the quilt. I don't know if this makes sense, but in the end, all the blocks will be the same size for putting them together.