Another Southerner here...LOTS of DFN round here! lol
I dont Mary Elizabeth is all that long. We considered it as Dh's gmother is Mary Katherine and gmother in Elizabeth. Most people that I know use the FN and MN spots for the DFN. such as Mary Elizabeth Smith: called Mary Elizabeth.
The longest DFN I know is Savannah Katherine. When I first heard it I was like WOAH what a name...but she is the daintiest little girl and it fits her so well!
My DD is names Carys Jo. I never intended for her to be called by both name but I almost always use both names when addressing her.
Personally, I would leave off a middle name.