The iPhone 7 has the perfect phone size but screen is too small because of the huge bezels. It could have fit in a 5" screen. The S7 with 5.1" screen is the ideal size for a normal smartphone. 5.2" screen on the XZs body is not ideal at all. The XZs could already house a 5.5" screen. For phablet, the ideal is 5.5" on a S7 edge body. The S7/S7 edge actually are the best pair of regular-phablet smartphones.
Screen size doesn't matter, device size matters.
For example Sony phones are full of bezels , so no matter the screen size the actual device size is much bigger. By contrast there are bezeless phones with big screen yet small overall dimensions.
According to studies best dimensions is below 130x65mm. I realize that there is no phones in that sizes (although in the bezeless era we could well have 5-5.2 inchers that are that small), but it's the best as far as comfiness go.
As far as amount of information goes then 6inches and above are obviously the best (they're basically tablets).
IMO a 5.5 incher that is below 140x70mm would of been the best. Of course it would have to be bezeless + palm rejection. When such a phone is ever released I'll prefer it, until then everything else has either too small screen or too big body...