[ilmare]

I think something between 3.5 and 4 would be ideal. Current 3.5 size is small for reading books(typical pdf technical book, page cropped to visible content). Even adding 0.25 inch would be a great help without sacrificing portability.

[dhughes]

Another thing to consider is in general the screen bezels of phones is getting thinner too as the screen glass size grows, a 3.5" (88.9mm) iPhone has probably a 1/4" (6.3mm) bezel surrounding the screen.

Someone with an iPhone would be used to the "hand feel" of a 4" (101mm) iPhone if the screen bezel were thinner.