Well, that last bit of info comes from the latest DeviceAtlas research report on the growing smartphone trends, which includes the most popular screen sizes for 2017, based on mobile web traffic data collection around the globe. It turns out that the larger the screen diagonal, the most likely it is that the country is in the emerging nations cohort.
That makes sense, as many of their more rural sections skipped the personal computer boom, and jumped straight into the smartphone revolution, thus the handset in their pocket (or, rather, backpack) is currently their main computing device, so the bigger the screen, the better. A 5.7" diagonal is hugely popular in India and Malaysia, for instance, with more than a quarter of the phones in circulation, while 5.5-inchers take another quarter.
It's the golden middle of about 5" that is the most popular globally, however, while tiny 4-inch screens are still the rage in many places, taking from 9% of the market in the US, for instance, to the whopping 16% in South Africa. These are mainly iPhones, of course, and, with Apple rumored to grow its average iPhone screen sizes this year significantly, the trend is clear, and the only way is up.