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.