Its not just the screen size, but the bezels that matter too. I man may say 4.7" is perfect size and 5" is too big. But then, HTC One is bulkier than Galaxy S4. Same can be said between Xperia Z1 and Galaxy Note 3. For a slight size bump we are getting a bonus of .7" inch and thats a LOT in mobile phone world.
I dont think we can define an ideal screensize. Human hands differ. American and Russian reviews of phones tend to show 4.7" screens to be ideal as they show how they can touch extreme corners with their thumbs. The same feat cant be accomplished by lets say, the Chinese or Philipino reviewers.
With Indian avg hand size in consideration, 4.3" is the ideal limit, but for Caucasians like Americans, 4.7" is the ideal limit. Even if we petition for compact super phones like iPhones, manufacturers will go for the demand of big handed Americans as USA is the main market for smartphones.
As a conclusions i'd like to state this--
1) Companies manufacture stuffs according to demand. As long as we keep buying huge phones, they will make more and more of it. In the end, the only ones we can blame is the consumers.
2) There is another thing called human adaptability. When we were in the Nokia s40/s60 era, i remember the Nokia n92 being criticized for its bulk. In today's age thats totally acceptable. The Noka n92 did sell well, and its owners adapted to its size. Instead of placing it in shirt pockets, they decided to let it rest in the same pocket where they keep their keys,handkerchief etc
As big phones started coming out, man has adapted to using phones with 2 hands.
One day when 6 inch phones become the norm, tailors & clothing companies may start stitching clothes with bigger pockets. That day we will open a thread saying "OMG SAmsung making a 7inch phone, 6 inch should be the standard !!! We want compact 6 inch phones!!!"
For me, 4.3" seems ideal, but when i used Xperia SP with a bulky body+heavy weight (despite a decent 4.6" screen) for a month, i got used to it,and didnt complain about the weight and size. It became MY PERFECT SIZE.
My yardstick for phone size increased, due to adaptation.. Today i use a 4" Xperia M, and i feel this is "perfect"and i know if i use the Xperia SP again, it will take me another month for it to become MY PERFECT SIZE again.