If you start to piece a backing, they should be larger pieces to cut down on the bulk from the seams. I did a Yellow Brick Road enlarging the blocks for the back of a Bargello quilt. Came out great. I normally throw some leftover blocks into the back of a quilt. You can sometimes save on the amount of backing fabric you use on a lap quilt by inserting fabric or using up leftover blocks. Good Luck !!