I have relatively small hands, but here’s a few photos: https://imgur.com/a/YIRAj – 100% of the screen is reachable, and on all below 4.9" also with a firm grip.
In contrast, on the 5.2" Nexus 5X, only with a loose grip is the left or upper part reachable, and the action bar’s up button is generally unreachable while holding the phone in a firm grip.
On larger devices, such as the Nexus 6P (currently none here, so I can’t take photos), it gets far worse.
Phone UI is designed for 4-5" devices, if you use larger, you have to live with that, but that’s your decision.