Increase width in catch evolution

Hi, After removing the sidebar from my default layout, I want to increase the width of the content area in my catch evolution site. I tried adding a custom CSS with

.no-sidebar #primary
{
    width: 1200px;
}

This worked fine on my laptop (full HD screen), but on smaller (mobile) screens the layout got completely messed up. I tried adding a responsive CSS file to my child-theme, but that did notwork at all. Can anyone explain how I can increase the width of the content area without losing the responsive features? Thank