The few times I have used flannel I prewashed it and it shrunk about 5%. One thought is to prewash a small square of your flannel, say 12" square and measure it after the wash/dry to determine how much it shrinks. Then you know how much bigger the piece of flannel should be. My second thought is just to make your flannel about 10" bigger (length and width) as that would allow for over a 10% shrinkage and that should be enough.