In the Layout section of the theme options you can setup the maximum width of the site, as well as the default options for the sidebar appearance in various places.
This is a responsive theme, which means as you view your site on smaller screens, the theme adapts its appearance. The 'Max Grid width' setting set the maximum width which your site's grid can reach. Eg. if you set it at 1200px, the grid will be 1200px wide on a 1920px monitor.
When the screen's width is smaller than this setting, it has no effect. Eg. on a tablet with a 768px wide display, the grid will be at a maximum of 768px.
Setting an explicit value in Pixels is recommended, although this field can also accept % values. Setting for example 'Max Grid width' to 100% will make the grid always as wide the browser's window.
The layout options mainly refer to the appearance of a sidebar
All these settings are valid unless they are overridden by individual settings in pages and/or related settings on the specific post type. For example if you select a 'Pages layout' view with a right sidebar, this will have effect on those pages where the individual layout settings (in the pages meta boxes) will be on 'Default' :