I use a desktop with a 24 inch LCD (1920x1200)
I currently use a pair of 27" LCD 2560x1440. And I'm itching for a 3rd screen.
As a developer, at this point, I consider a pair of matched 24" screens to be the minimum for any serious work. And I fully expect that after getting a 3rd screen I won't be able to go back to 2.
I don't see how anyone is satisified with one after having two. Visual Studio in full screen with multiple code panes, watch windows, etc etc on one... the application, along with my email, word/excel/pdf documents (specifications/documentation/references, etc etc), browser, slack/skype/etc in the other.
A 3rd screen would let me have more information immediately accessible.
As a gamer, I opted not to get a 4k screen last cycle because i didn't have a card that could push the pixels. Now, I'm thinking I'd rather add a 3rd 27" and try some triple screen gaming rather than a 4k screen. OR maybe I'll do a 4k in the middle and use the pair of 27s on the flanks. But I like having matched screens enough I'll probably skip 4k for another few years.
As an aside, I have a 2015 13" macbook pro as my laptop for email, browsing, programming cisco routers, etc. And its fine for that. I like it because its light and portable, but i don't even try to do any real dev work on it beyond answering email.
I also hate trying to game on it -- it rapidly runs too hot for my lap and fingers and the fan noise is distracting. (even for stuff like ToME or DoomRL or SoTS the Pit or Darkest Dungeon which really shouldn't be that demanding)