Need help changing other dimensions of my website

Hey y’all
I’ve finished designing my website for big monitors, but now I ran into a problem when I tried editing the website for example the IPad.
Due to the Header not fitting on IPad, I tried changing some Heights and Widths on it, but for some reasons it changed the standard dimension aswell.
Now I don’t know how to make changes on the Ipad, without changing it for the standard pc website.