A consensus is about all you can get about where lies "Little Dixie." It appears the area to which this name applies has never been precisely defined.
I was delighted to find Robert M. Crisler's scholarly January 1948 article, "Missouri's 'Little Dixie,'" in volume 42 of the "Missouri Historical Review" many years ago. I had hopes that Dr. Crisler would clear this up. No such luck. The whole substance of the article was that nobody agreed on the precise boundaries of this nebulous area. He stated the name was given to the region after the Civil War, and he maintained it referred to the inhabitants' political persuasions of that time along with the inhabitants being born in the Upper South. He quoted historian Paul I. Wellman as writing also in "MHR" in 1941 that "Little Dixie" was 14 counties, but Crisler differed from Wellman since the latter omitted Monroe County. Dr. Crisler also took a poll of a number of prominent Missourians whose counsel he valued and gave these results of their voting about which counties they felt belonged in "Little Dixie": Audrain-17, Callaway-17, Monroe-17, Boone-16, Howard-15, Randolph-15, Pike-11, Ralls-11, Chariton-9, Shelby-8, Lincoln-6, Marion-6, Saline-5, Clay-3, Platte-3, Ray-3, Lafayette-2, Carroll-1, Jackson-1, Lewis-1, and Macon-1. Dr. Crisley himself committed himself to the following counties as being in "Little Dixie": Audrain, Boone, Callawy, Howard, Monroe, Pike, Ralls, and Randolph. He agreed that Chariton, Lincoln, Marion, Saline, and Shelby could be named in the region and wouldn't get an argument from him, as well as Clay, Platte, and Ray. He disagrees with those who place Lafayette County in "Little Dixie."
There. Clear as mud.