I've used various dual monitor configurations. I currently use a 24" and 27.5".
I've used: 17", 19", 23", 24", 27.5", 30". I don't care for a monitor under 22" or so as a single (I've used 19" alone and as a second and wasn't happy). I am not a programmer/IT. I do what the vast majority of people do (without the gaming portion), but also view building plans for work. The large screen size is really needed for that purpose. Often, I just use the 24" alone.