Most quilters usa a single fabric for the quilt back, thus keeping the focus on the quilt top. The quilt back needs to be larger than the quilt top to allow for fabric that is taken up during quilting and for stabilization when using a quilting frame. Always add 6" to both the length and width measurements so you have an extra 3" of fabric all around.
If your quilt top is wider or longer than your backing fabric width, you'll need to piece the backing. If seams are necessary, you need to decide whether you want the seam on hte back to be horizontal or vertical.
If your quilt is 40" to 60" wide, horizontal seams save on yardage. If your quilt is wider than 60", you may use one or two vertical seams.