Any monitor is viable. And if go hdmi or any other connections to the monitor depends on the graphic card u use
personally i prefer a relativ big screen for the setup.
And i use five one big four small under the big one.
This is for being able to monitor the other chars behavior and if someone
is not where it should be. Melee running far for example.
If u prefer fullscreen ill guess a second small monitor for the rest chars is anotherway to go.