If the tablet market follows the smartphone trend, expect Apple to stick with a 9.7" display for a while while Android manufacturers roll out 13"-15" iPad competitors. 

you can easily hold it and use your thumbs on the keyboard
you realize you can do the same with an iPad, right? Just spread with your thumbs to split the keyboard. Then all the keys are within reach, just like on a tablette.
There is no reason to reach for the center. Maybe you misunderstand? Watch this video, or just google iPad split keyboard. It exists specifically so you can hold the iPad and type with your thumbs with no issues reaching all the keys (cause there are none in the middle).not without the tendency of my palms to touch the/screen when reaching for the center, I have small hands.
Many Android makers are also about the quality of the screen. I'll gladly take the blacker blacks and incredibly awesome contrast of the OLED screens over a retina screen and its lousy contrast and greyish blacks.
About that superior contrast and blacker blacks... Enlighten yourself via Google / Wikipedia. Try the term "OLED MURA". Also Google "OLED Fading Over Time".
I was using OLEDs, in all likelihood, before you even knew what one was. Not nearly as epic as people would have you think. Don't get me wrong, LCDs have their problems, but I'll take retina (or any other high density LCD) over an OLED.
Many Android makers are also about the quality of the screen. I'll gladly take the blacker blacks and incredibly awesome contrast of the OLED screens over a retina screen and its lousy contrast and greyish blacks.
Unless you go legal size (for viewing legal sized docs natively) I don't see the need for anything bigger than 10 inches really, unless for specific artistic creation reasons and Wacom seems to have that covered.
I used OLED screens and a retina. The mura is only a problem to people that desperately want the retina to be better. OLED produces infinitely better images than retina at this point. Hopefully Apple can address that in the future.
At 8.3x11.7 A4 is closer to the North American "Letter" size of 8.5x11.Legal is fine, but most of the world uses A4 size paper which is slightly different than the US.