Dual 24" monitors (decent quality IPS), one uses an HDMI KVM to switch between different/retro systems, Recently upgraded from VGA to HDMI switchbox, the difference was worth the cost to buy some [X] to HDMI adapters. Now there's enough inexpensive ones to cover all the bases.
The benefit for me is I use the main system to look things up and then switch to the target system and I have the main display as a reference and the other as well,
Used to have the side monitor as the KVM one, but find I work better with the switched monitor the one in the center, as a side display I would get sore/tired neck from always looking to the side. With it center I am more comfortable to concentrate on the task at hand on the other system I have up.