Based on personal experience, I think the upper bound for comfortably useful size at normal sitting distances is probably about 32", and even then I think there'd be better returns on adding vertical pixels to a ~27" monitor. A modern equivalent to the old 16:10 30" 2560x1600 monitors (ideally 2x scaling 5120x3200) would be great for example, but one could also imagine a 4:3 or 5:4 monitor with the same width (~23.5") as current 16:9 27" monitors.

I'm still rocking a couple of 30 inch dell 2560x1600 monitors. They're about the perfect size and not dealing with scaling in Linux is nice. I'd pay a ton of money for a modern equivalent.

Same! My employer offered a choice of 32-inch and 40-inch monitors. I “upgraded” from 32 to 40 but I regretted it. I just don’t make use of the extra horizontal space effectively.