I'd use a 1/2 seam or 3/8, something bigger than a 1/4. Really depends on how close the weave is.
Also, wash those flannels in hot water and dry on hot dryer to shrink up as much as possible. Ask me how I know to do this!! I like the wrinkled old fashioned look but not as much as can happen is you don't rewash flannel. Just my thoughts.:)