It may well be. I have a couple of books on Star Wars ships and vehicles, and one of them does list the SSD at 12.5 km. I suppose I will go with that.
Going back to the modeling side of things, I can tell you this much....ain't no way, no how am I gonna try to make a "full scale" super star destroyer. Ha ha. 125 full grids?! No freakin way it would ever work. LOL!
I'd actually have to make an SSD scaled to my 16 square (not 16 grid) ISD. 125 squares will be easier to pull off, and I can still make the ships look epically sized.