Your backing fabric must be at least 4"-5" larger on ALL FOUR SIDES of the quilt. So if your quilt top is 45" x 45", plan on sending a backing fabric that is 50" x 50".
Your backing fabric can be ...
ONE large piece of fabric - using extra wide fabrics available from quilt shops and specialty stores (generally 108" or 118" wide).
Standard 44"/45" width fabric SEWN TOGETHER to make a piece wide enough for the quilt top. When planning out how much fabric you'll need, remember to keep your seams horizontal so that they lay better as they come over the poles of the quilt frame.
Pieced backing that you MAKE YOURSELF. As with the selection above, when planning this type of backing - try to keep you seams horizontal so that they lay better as they come over the poles of the quilt frame.