Apple said 4 inches was the perfect size. Back when the iPhone 5 came out.
Apple in 2012: iPhone 5
"Anyone can make a larger smartphone display. But if you go large for larges sake, you end up with a phone that feels oversize, awkward, and hard to use. iPhone 5 features a 4-inch display designed the right way: its bigger, but its the same width as iPhone 4S. So everything youve always done with one hand typing on the keyboard, for instance you can still do with one hand. On a larger canvas that lets you see more of every web page. More of your inbox. More events on your calendar. Even more apps on your Home screen."
What happened
Apple was wrong.