About screen sizes. Apple says that having a screen that's too large will decrease efficiency - and according to this article by Ars Technica, it certainly seems to be the case. However, Apple might have miscalculated the "perfect size" that balances screen real estate and usability. The attached chart shows that user efficiency declines sharply as the screen size increases from 4.7" to 5.3". 4" screens aren't especially more efficient than significantly larger screens, such as the 4.7" panel found on the HTC One. Granted, this was a "small scale study" which did not explicitly focus on single-handed operation like Apple did, but it was still conducted at the respected Human Interface and the Management of Information conference at Las Vegas. I guess it's something for Apple to consider, as many users (such as myself) would gladly jump for a larger iPhone, somewhere in the 4.5-4.7" range.