I've done a couple of scrappy backings. One I measured and actually had a block in the middle of the backing, the other was just random size rectangles of fabrics that coordinated with the front. But in both cases, I already had at least a yard of the main backing fabric.
I've seen some, though, where the pieces were smaller, pieced randomly in a sort of "crumb" style, although the pieces weren't THAT small (because that would be craziness).
My mom's first quilt was actually two quilt tops, one on the front, one on the back.