It varies from task to task, and operating system support is a confound, but basically it comes down to how often you have to scroll or switch windows to do whatever you are working on. For example, a print designer might do best with a single monitor large enough to hold the full document they’re working on at comfortable resolution while a programmer might be limited by having their editor and something like a debugger, browser, simulator, etc. simultaneously usable so two monitors might be better than one big one until it’s so big that those all fit.

https://www.jonpeddie.com/news/jon-peddie-research-multiple-...

https://www.microsoft.com/en-us/research/wp-content/uploads/...

http://portal.acm.org/citation.cfm?id=1375718

http://infovis.cs.vt.edu/oldsite/papers/Shupp-HCI.pdf