Two years ago, I pulled together a look at the most common mobile device form factors and how people were using them (portrait, landscape, etc.). At that time, 67% of mobile devices had 4"-5.5"screens. Since then things have changed significantly.
Over the course of just three years, active smartphones with 5.5" to 6" screens grew from 7.5% to 43% according to Scientia Mobile's panel of mobile Web browsing activity.
And in 2017, 5-7" smartphones became the majority of native mobile app use in Flurry's sampling. That's some "big" changes as larger screen sizes not only impact design but time spent on devices as well: