Originally Posted by quilting cat
If you choose backing first, use the width of that fabric for one dimension. (Flannel, for example, is usually 42 -43" wide.)
For "lap" quilts, you want the length to be between 54" (1 1/2 yd) and 63" (1 3/4 yd).
This is how I do mine. Saves a lot on the backing fabric.