My rule of thumb has been to have enough height. Then as much width as you have space for.
According to most ergonomic manuals the top edge should not be higher than eye height, however.
And the sweet spot for PPI has been 100-110. That would make the ideal vertical span 1440 pixels.
So, the screen setup should be one or several of:
22" 4:3 1920×1440
24" 16:10 1920×1200
27" 16:9 2560×1440
34" 21:9 3440×1440
However, with ageing eyes, I think I'd like to try placing the screen further away, and then perhaps using even larger screens.
I feel that my 2' deep desk (one that I've had since childhood) is not deep enough any more.