I would probably add that extra inch in length, maybe even one and a half.. I'd also suggest pre-washing the flannel and tumble drying it, both on high, 2-3 times to get the shrink out of the way. If the stable knit has any cotton content I'd do the same, but always at least once before cutting. You do not want a onesie.
Maybe take a tape measure to her, neck to crotch and also the pattern to compare. If she side sleeps and likes to curl up a bit that takes up extra fabric which is why these things are usually knit for more give if the child is a little contortionist.