It took 2 months but I finally put down my vote for this thread. 11"
After 5 years of trying to make the 12.9 my "all-in-one" solution, it has become very clear that Apple has no intention to optimize the larger iPad interface, utilize this excessive power or even allow their own apps. So I recently switched to an 11" and I love it. While I do lose out on that big screen, I do gain comfort, portability and an interface that feels appropriate for the side. In other words, I now have a tablet.
After 5 years of trying to make the 12.9 my "all-in-one" solution, it has become very clear that Apple has no intention to optimize the larger iPad interface, utilize this excessive power or even allow their own apps. So I recently switched to an 11" and I love it. While I do lose out on that big screen, I do gain comfort, portability and an interface that feels appropriate for the side. In other words, I now have a tablet.