The best size for me is 5.5 inch atm but if they bezels are slim like rumours are suggesting, the 5.7 inch version should be more than good enough.
But I aint a buyer though, it costs too much for what you get.
At least 6" in compact body, so the 6.3" sounds good. The bigger the better for me as long the phone size remains pretty same as my Nexus 6 now is. Fits every pocket easily, but a bigger screen would be nice.
I want sub 5in... side bezels are too big at any 5.5in even if you have no bezels, it's still too wide.
They should go even bigger than 6,3"! I think if they can manage to pack a 6,2" screen in a Note7-body, they also can fit a 6,6 to 7" screen into a body with the size of a Galaxy C9 pro or LeTV Max, which has 6,3". They also should ban small screens below 6". No more tiny phones with tiny screens and highend hardware. Maybe they can put the crap hardware in those small phones.