I have normal sized male adult hands. I can easily reach the top bezel area of the SE body. Obviously the bottom bezel area is easy to reach with a thumb. The 5.4" iPhone should be only slightly wider and taller than the SE. I'm confident that any adult male at the very least will be able to easily navigate all dimensions of this phone with one hand.
Are you also taking into account swiping gestures from the top and bottom edges?
The SE (and earlier) was designed for this. If it was easy (for most people) to do this on a bigger screen, why wouldn’t the original size have been bigger? And what about this iphone 5 ad?
It may be easy for you, but I also have normal sized hands and it’s not easy for me. And according to this ad, apparently Apple believes that for most people the 5/SE screen size is the ideal for one-handed use.
Also take into account, it can’t be just for average full grown male hands. It also has to be for females with smaller hands.