But first you are comparing bananas and cows. Amoled is over at http://en.wikipedia.org/wiki/AMOLED and then you want to compare it with what is IMD (in my definition) a display with more pixels than we can see at handheld distances.

As such your question explodes on the launch pad.

Care to try again?

On your second question, the buyer may need one in their shirt or other pocket and a vertical app may need some 10 inch screen to do the work. So again, not one size is the right size but many. Apple, Samsung, Amazon learned this and you can see their selections.
Bob