Screen-sizes are not the driving force for designers. Humans are.
“I’ve read many articles lately that tell us the new iPhone 6 series will force all of us to change the way we approach UI design for mobile phones. Well, it may for designers who still focus only on iOS and pretend the rest of the world does not exist. But large-screen, portable devices have been around for a long time, and those of us who design for every platform have been considering them in our designs since at least 2011. In fact, larger, touchscreen, handheld devices were available as far back as the Apple Newton, with its 5.25-inch screen. So far in 2014, about a third of all the smartphones sold have screens that are over 5 inches on the diagonal – even before Apple got on the large-screen bandwagon. Not just worldwide sales either. Even in the US, large-screen phones are a huge force, so you can use information about how people use them today.”