What is the best screen size to design for?

Based on the Worldwide Screen Resolution Stats (Nov 2016 – Nov 2017) chart above and the charts below, these are the resolutions you need to be aware of in 2019.

360×640 – 21.54%
1366×768 – 12.85%
1920×1080 – 7.76%
375×667 – 4.94%
1440×900 – 3.32%
1280×800 – 2.67%
These resolutions take into consideration desktop, mobile and tablet resolutions.

Source: https://www.hobo-web.co.uk/best-screen-size/