You could certainly do a 12-inch block; however, you probably want to take into consideration the combined limiting factors of batting, distance between quilting lines, and the shrinkage characteristics of flannel. The usual X in the middle of the block might leave too large an area of unquilted sandwich, allowing the flannel to shrink disproportionately to the batting. In addition to the large X, I would quilt a box in the middle of each block to provide enough quilting for the larger expanses.