Honestly as a buyer I'd prefer 8x11 (standard letter-size) for prints, especially if I'm at a convention; I probably wouldn't even notice that a 9x12 print was not 8x11 until I tried to fit it into a protective sleeve and realized it was just a little bit too big, and then I'd have to carry it around all alone for the rest of the day (or have it sticking partly out of the binder) and risk damaging it. There's no shame in standard sizes! They also tend to be cheaper to print.
For me size really does make the difference though. For things that are cute and simple (and in particular prints where the main selling point is humour, like a silly visual gag), I prefer smaller sizes-- postcards and the like. For things where the main selling point is intricate detail, or beautiful lighting, or some other visual component, then larger sizes are better because you can really see it.
I've kinda noticed this bearing out when I bring my own work to conventions; I have some prints of elaborate paintings that have literally never sold at their smaller size, despite it being half the price, but I can barely keep the big versions in stock. For simpler pieces (in particular some of my cel-shaded stuff), buyers at conventions seem to prefer the smaller size.
