What the hell ?? -_-
No, really no, this is way to much big ....

What is the average thumb/hand size you take ? During your researches, did you take in account that everybody has its own preferences about the hand positions, and some people rather prefer to have thumbs not streched but crooked ?

I hope there will not be any combo keys which requires to have a thumb from one side which should reach a button on the other side of the keyboard (not a problem with smaller keyboards). In addition, having a smaller screen can still be fine with big hands but be more convinient for people with smaller hands, which means more potential customers.

A bigger screen also means more battery consumption, less solid, less possibility to use the screen with only one hand, and it will not be always possible to keep it in the pants pockets.
On an other hand, with a Nokia N900, I didn't read any comment saying the keyboard was to much small.

On my part, my favorite screen size is 4-4.5", and I have an average hand size for my country. 5" and 5.5" is big but well, since it's the only device which interrest me, why not. But 6" is waaaaayyyy to much.