Just last week: My task was doing 3840x2160 screenshots on a 1920x1080 screen 😉
My luck was that it was for a web application. So I found out for Firefox:
Ctrl+F2 (opens Firefox cli)
```resize to 1920 1080
screenshot --fullpage myfilename.png --dpr 2```

the --dpr param was doing the trick for resizing the screen to pixel ratio from 1:1 to 1:2