Gorgeous quilt! If you have any scraps, just add them to the back to make it large enough. It doesn't have to be a sophisticated pattern, but it will make the back more distinctive. Make the back 2-3" larger all around if you're quilting it yourself, but if you're taking it to a longarmer check to see what her requirements are (usually about 4" all around). Cut the batting about the same size as the backing.