Varies from person to person.
On a desktop I use 26" 1920x1200 resolution monitors. 24" is a bit small and I do not like using 1080p monitors, then again 1920x1200 are no longer manufactured now or they are extremely rare.
On a laptop I can play games on a 14" or 15.6" monitor since I have both sizes. I would prefer a 1920x1200 resolution screen, but they simply do not exist for laptops. Though a handful of tablets use that resolution as well as some MacBooks.