A:
I would entirely determine length of rod by comparing it to the height of the angler. Instead, look at what you are going to be doing with it, where you'll be fishing, etc...
If she is going to be doing lots of flipping and pitching from bank - 6'6" -from a boat, a 7' might be OK, just general casting - either size is fine. Her casts might be slightly more accurate with the 6'6" but a hair longer with the 7'