Here is most commonly used monitor resolution worldwide by statcounter. Most used Desktop/PC/Laptop monitor resolution is 1366*768. 2nd most used resolution is that old one 1024*768. Yes, it is still relevant. I think those who are into web development may remember it for sure!
For this Web Scripting blog, most used screen size is also 1366*768 and 1920*1080 is at second position.
In Mobile, most commonly used screen resolution is 320*480. 2nd most used screen resolution is unknown according to StatCounter and I am not sure why they mean by it. 240*320 can be said 2nd most used resolution on mobile device for design purpose then.
I am sure, if you are into web development or into design of UI, then this stat about screen resolution is something you want to remember!