This morning I was using my new 12.9 on the subway, and I was so appreciative of having a large display without having to deal with the bulk and weight of a MacBook Pro (I sold my 15" last year). It's all a matter of perspective -- so many people compare the 12.9 to smaller iPads, but if you remember how it felt to lug a laptop with a similar-sized screen, the big iPad feels wonderfully light and easy to carry around.Aside from regardless how tall somebody is or how Large the iPad is, eventually you adapt to it. Most who initially graduate to a larger display, at first it's awkward or cumbersome to adjust to a larger display, but overtime you realize you could never go back to anything smaller than a 12.9 iPad Pro. It also depends on how mobile somebody is and what they want for their convenience.
I'm not getting into a operating system comparison here (although I think that iOS 11 will make that point closer to be moot), just talking about portability.