The Mac has decent global scaling options in settings. However I tend to leave it at native scaling and choose a decent font for Emacs (Inconsolata) and tend to have the font large enough to have about 300 columns (100 in 3 windows wide).
On Windows I find the font rendering and scaling not quite as good, so I think it's more like 270 columns (90 in 3 windows wide).
I also bind shift-mouse up and mouse down to increase and decrease font size (I think this copies Visual Studio) for showing code to others or for rare occasions when I get strain.
Other advice I'd have is to splurge on a proper full eye exam by a professional optician at least every two years and have specialized glasses just for coding - the focal length is set for the monitor distance (though they have a distance focus at the top 25%). They took a little bit of getting used to, but they really helped. They were also pretty expensive, but again this is your health and your work, so spend the dollars if you can.
I once had shoulder/neck issues, and had several trips to the physio - he was the one who advised me to get a proper monitor arm and chair - so I took his advice and havn't had any issues since.
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