I use the BenQ RD280U. It’s as wide as a 27” 16:9 and as tall as an 32” 16:9. I really like the aspect ratio and I’m using two of them.

Eizo makes a square 1920x1920 monitor:

https://www.eizo.com/products/flexscan/ev2730q/

... available on Amazon last time I checked ... they also make a square 2048x2048 monitor for ATC:

https://www.eizoglobal.com/products/atc/sq2826/

... although I think it costs $5k or so ...