Why would eldery users prefer smaller size screens?
I am a bit worried about Apple's desire to remove the bezel since that will cause some problem and iBooks likes to flip a lot of pages if your thumb strays.
1) [edit] the iPad pro is just a no go because of its size, and the iPad 9.7 is too big for a lot of children and some adults. Remember, a lot of activities are hold it up with one hand for long periods of time while tapping with the other.
I think that is where iPad is heading. Some argue Apple is obsess with thinness, personally I think both iPhone and especially the iPad has a long way to go before it is Too Thin.