Trialled a 20-inch monitor (2005 Apple LED Cinema Display) as a second screen with my 4k iMac 12 hours ago in a Zoom meeting. I had to disconnect it as the screen refresh rate was so slow that image ghosting was evident. In any case the max resolution is 1650 x 1050 & that don't cut it these days. It's OK for occasional use hooked up to either a 2003 15-in G4 PowerBook or a 2005 12-in G4 PowerBook.
Local Officeworks (Office supply store chain) has Acer 21.5-inch LED 1080p monitor at an attractive price: I'll get one when the store opens in around 5 hours.