This is really a complex question. One of my dancing friends just returned from a cruise on Seabourne, and they had one barely adequate dance floor, but oftentimes it was taken up by food service or some other non-dance need.
The Queen Mary probably has the largest dance floor afloat, but there are a whole bunch of dance hosts, and people get up to sway, not to dance.
Dance cruises, of course, always have too many dancers on the floor. I want the floor to myself!
I was on a Princess cruise a few years ago, where they had 2 dance floors, one for swing and the other for Latin or ballroom, and we ran back and forth from one to the other all night for the entire trip. That's what sold my wife on cruises.
But then, we were on the Golden Princess last year, and the floor was small, oddly shaped, and often preempted by other needs like Bingo or Trivial Pursuit.
Jerry