Using the Xperia Z1 Compact and Samsung S3 as reference points, I would rate one-hand usability of the Z1 Compact as 10, the S3 as an 8 and the Z2 as a 6 or 7.
However, there are ways to make it easier to use the Z2 with one hand. If you have Nova Launcher installed for instance, you can swipe downwards anywhere on the homescreen to pull down the notification bar; no need to reach all the way to the top. And if you install the Touchpal keyboard app, the app allows you to shrink the keyboard to one side of the screen for enhanced one-handed usage.
By comparison for example....
HTC One M7 8
Nexus 5 8 or 9
It's the added few mm of width that do me in....I don't have small hands at all and I ever feel on the verge of dropping it trying to reach around one handed. Having said that, I have quickly adapted to use two hands for things I used to use one. I have no trouble texting with one hand.