I don't have a high-DPI display, but the display size shown on about:support (which I suspect is also what is being transmitted via telemetry) seems to properly

I don't have a high-DPI display, but the display size shown on about:support (which I suspect is also what is being transmitted via telemetry) seems to properly handle Window's display scaling – I've experimentally changed it to a value > 100 % and the display size reported by Firefox remains unaffected, only the scaling changes accordingly. Plus if you hover over the diagram, you can eventually discover some screen resolutions > 1920x1080, although all of them in the low single-digit percents. (And some might be lumped together underneath the "Other" category.)