Hi @atreidesp, thanks for the question. You can add the custom css media query to the Header in your custom code section to make your site 1200px in published mode.
So you can make a site 1200px via custom css, but you cannot see the media query changes in the designer or preview modes. We do not have built in 1200px breakpoints, anything 991 px and higher is considered Desktop mode (including ipad horizontal view for example).
I hope that helps 
Cheers, Dave