I have many times pieced my backs to making it large enough. If I have any left over blocks I'll put those together to make a row. I made a Row Robin quilt this year where I enlarged the blocks I'd made to put on the back. I also used large sections of fabric colors I pulled from the blocks on the front. I did draw it out on graph paper first, but that was quite general in size. I know some long armers don't like a pieced back, but I fmq all my quilts, so it doesn't matter to me.