The diagonal measurement from corner to corner on the iPhone 7 is roughly 6". So if Apple releases a bezel-less 5.8" iPhone the footprint is more or less what we have with the iPhone 7.
Still, there will be some bezels at least at the top.
And see here http://www.apple.com/lae/iphone-7/specs/
It's not 6, it's just 5,4