Are you a designer? A web design agency?
What size should you design for?
We basically start with the desktop resolution, 1920px width, with containers from 1200px to 1400px. After that we are designing the mobile at 375px, and if the case the tablet, at 1024px.
This will depend on your type of design of course, and on the front enders. A good front ender will match the 1920px width full design for all of the other screen sizes, but if you can help it from the start, you should keep in mind the most important 3 breakpoints.
More info you can get on Deisgn Rush: