How to make a fullpagescreen mobile version of the site by getting a photo in a higher quality?

Question

Photo

Good afternoon, I make site mockups, in order to get a mocap in 4k, mobile view photos must be loaded in 1080p.
I installed the plugin for chrome full page screen capture and insert the mobile size, and it takes pictures but has a width of 375 (like the iphone size) , and why I can’t embed (terrible quality)