Clearly you need to 3D scan your hands in your normal typing position and get a full size 3D model printed for Chen so he can check it for you

For what it's worth, I can reach 2/3 of the way across my Fairphone 2 in landscape without feeling like I'm stretching. That's 5" with quite a big bezel - 143mm height. I'm 195cm tall so I have big hands, but I think the intention is to have a smaller bezel on the Chenphone and you only need to reach 50% of the way across with each thumb, so...