I do not doubt that Apple not only did a lot of research on the perfect screen size, but they probably went overboard with it. They are nuts about these kinds of things. When I worked for Apple, I would always hear stories like how they created hundreds of different colored casings for the iPod mini and spent an entire week just to figure out which 5 to pick. Any other company would make that kind of decision in a matter of hours.
I'd argue it's that attention to detail that has made Apple great - Apple doesn't settle for merely competent decision-making, and this is plainly visible in the end product.
I now work for a company that has some of the highest-rated and most perennially popular apps on the app store, and the focus on details is insane. We will go in and fix a tiny animation inconsistency that would last barely half a second on screen - but people perceive this polish, whether they are conscious of it or not, and it crosses that gap between merely functional and actually pleasurable to use.
