The standard size is the correct one for the 2012. Just like the earlier models, the measurements contradict this, but if you actually ever see the wide bag, you'll know why. It's WAAAAAAAY too wide. (I was lucky enough to have a dealer willing to order both, and allow me to compare the fit before buying.)
The only real difference between the 2011 and 2012, applicable to this topic, is that the passenger pillion is thinner, so there'll be no way to prevent this from rubbing on your fender (it doesn't really work on the 2011 either, long term). I HIGHLY recommend getting some clear film/tape paint protection for your fender.