Optimal Viewing Distance and Monitor Size
Not considering screen size, you’d want to sit at a distance that gives between 30 - 40 degrees horizontal FOV for an optimal viewing experience according to the recommendations of some commonly cited associations.
Trying both I personally prefer the tail end of that range for a more immersive experience. Which means given a ~2ft (roughly measuring was 50”) viewing distance to my monitor, and a 16:9 aspect ratio that would translate to a monitor size between 31” - 42” to fit in the ideal HFOV range.
Minimum Resolution
One recommendation I’ve seen is to use PPD (Pixel Per Degree) to account for pixel density and HFOV into a single metric. In this article it mentions that 20/20 vision corresponds to 60 PPD, which seems like a decent metric to target.
This would mean for the prior 30 - 40 degree ideal HFOV range, you would want 1800 horizontal pixels for 30 degree and 2400 for 40. Going further and using our 50” viewing distance, with a 16:9 aspect ratio we select a 38” monitor which fits between our range of 31”-42” recommendation and gives us a HFOV of 37 degree. This would then need 2220 horizontal pixels to get a 60 PPD.
That would mean a 4K UHD monitor with a resolution of 3840 * 2160 for 16:9 aspect ratio would be slightly below the 60 PPD threshold we need for our 38” monitor and actually come in at ~58 PPD. Which realistically is fine since I don’t have 20/20 vision anyways. This also lines up with this TV Monitor viewing Distance Calculator which obscures the PPD for you.