My portfolio - just to be awkward - is 9x9, but mainly because I got a good deal on a really nice p/f box of that size. My last lot of exhibition prints were 12x12, but I'm thinking of going smaller next time around, or possibly mixing it up with a few larger prints, depending on the hanging situation...
I do think that these choices do depend a lot on the work and the space in which it's presented, and also that there aren't any rules for what will look good. I've been today to see the Rene Burri exhibition in Manchester, and they'd used just about every method going - salon style, huge digital prints, triptychs, a wall of images edge to edge, contact prints in big frames - and, really, all of it was effective.