What are the ideal dimensions of a video or film?

The resolution of a movie screen is measured in pixels,

Computer example

HD = 1920x1080px
2K = 2048x1080px
4K = 4096 × 2160px


Telephone example

HD = 720x1280px
2K = 1080x1920px

Look at the technical specifications of your phone or your computer, what resolution does the screen have?
For example, if the resolution of your screen is 720x1280px, it is totally useless to buy or download a 4K video!