Found the above in an old article here, (modified it slightly with some measurements for the benefit of non-iSheeps)
Following the source links to this and this, does not tell of what population base this is the “average” though.
As the source explains you can of course shift the larger device up and down in your hand to get to a different area.
But though possible this is inconvenient, and it is thus more natural to use these phablets with two hands, unless you really must stick to one hand (say holding a bag or trunk in the other)
The images are shown with right-hand usage.
For me as right handed it is natural to hold the device in the left hand, and operate with the right index finger or thumb for two hand operation, and thus it is also more natural to use the left thumb for one handed operation, already holding the device in the left hand, rather than shifting it to the right hand. But of course this might be due to me being almost ambidextrous, where others have one very dominant preference.
So if people got small hands they would use two hands anyway, and then they could just as well go for a larger phablet.
Is that was is driving this chase for the huge phablets?
Of course it’s NOT just the display width that matters, the side-bezels does too. To be more precise, we need to reach over one side bezel plus the full display width, so that is the primary limiting factor.
Of course the device thickness also matter, as well as the total width to keep a grip. So the red+blue in the below
(holding in left hand seen from below, or right hand seeen from above)
The shifting of a device mentioned above also leads to the discussion of aspect ratios.
I would say that for a large two handed device OR for a small pure one handed device, 16:9 is ideal.
BUT for the intermediate ones, it makes a lot of sense to have elongated devices where you (stretching) can still reach across, and can shift it up and down to reach larger area, still keeping a good grip. (like the S8-/S9- with 18½:9).
On the other hand the elongation seems like a stupid idea for the two handed ones like S8+/S9+ or the Note models IMHO.