You would just change the dimensions using the browser devtools no user agent faking needed

I'm not sure dimensions are all that's different?

Your website might want to present a different interface for people using mouse and keyboard than for people using tiny touch screens? Even if the number of pixels in the browser window is otherwise the same.

I think Wikipedia redirected based on user agent, but yes, whatever, point is if you're a developer you can use the browser devtools to simulate whatever you need.