Your backing will need to be at least 6-8 inches larger than the quilt top lengthwise and widthwise. If you are going to piece your backing here is a little guide to help you with yardage requirements.
If you are piecing 44 inch fabric you will need two equal pieces for twin and double size and you will need three equal pieces for queen and king. Please make sure to trim off selvages before sewing the pieces together and press the seams open. If you have washed the fabric for the pieced quilt top please make sure too also have the fabric washed and ironed for the backing.