the backing needs to be larger than the quilt top---gets trimmed after quilting...
so...if your flannel is 44" wide(?) you could take 2- 2 1/2 yard pieces and sew them together....your back would measure 89 1/2x 871/2 (assuming using a 1/2" seam allowance) which would give you enough in both directions (more is good--you can cut off the excess after quilting and add to your stash...less is not good, you could wind up being *short* by the time it is quilted)