Responsive Web Design based on physical size?

The current practice of using media queries via pixel-thresholds is fine, but what if someone decided to use their 50-inch 4k tv as a vertical monitor and actually sit really close to it?
I could see that happening if their work revolves analyzing or referencing really longs lists as constant scrolling would get tedious.

How then, would you style a webpage that only has a really long list? Others may visit the site via a smartphone, and since resolution is comparable between all types of devices now, how could you differentiate the two?

Do you have to rely on user-agent headers and such?