Hi Debby,
First of all I would wash the flannel to get the shrinkage done.
For piecing I cut it and stitch down the middle. YOu could even take some fabric and make some blocks for the back where it meets in the middle of the backing, something like that.
I've heard that piecing the back can cause stress on the seam but it's never happened to me and I've made many quilts.
Hope this helps!
Sue