Size really is subjective. I specifically bout the 12.9, with more of an eye on "close to laptop replacement", and with iOS11 firmly in mind towards that end. Granted, most of what I do is more basic productivity and content consumption, but I have the possibility for some expanded performance if needed without hauling my MbP around on business [especially if I need to take my Windows laptop for gaming, etc]
Sure it's not quite as portable as the 10.5, but the extra screen real estate can be a real boon depending on the application, and it still fits on an airplane tray table . . something my 15" MbP TB or Alienware R13 R3 really don't do.
Of course the other big thing is that my eyes have never been that great, and between glasses and contacts larger screens have always been a bit more comfortable for me to read on