Well, if i hold my current 5" phone in landscape like i would type, with the bottom corners firmly in the center of my palm, i can just get my thumbs to touch each other.
If i position the sides of the phone to just at the base of my fingers i can reach the entire screen with one of my thumbs... add another 0.5" and i would have to hold my phone halfway up my fingers to be able to reach the entire screen.
If yours reach 2/3 on a 5.5" (you do mean landscape right?) in a normal typing position... how huge are you?

When i first learned about this thread and a new upcoming landscape slider i was pretty excited about it... then i learned it would be a 5.5" device, now i'm not even sure i even want it (since it wont be available in shops cant really get a hands on to try it to see if its workable).
Seeing as i dont even want my touchscreen only phones any larger than 5" (cause i want to still be able to use the phone in portrait when i dont have both hands available), why would i want a phone with hw keyboard that big?

Considering its gonna be a 5.5" device anyway might just as well cater it all the way for big people, i've lost my interest, dont even know why i bother replying...