Configure at Admin → Extensions → Template Manager → rt_fracture, then go to Layouts tab and set the varying Mainbody/Sidebar layout possibilities.
Note: If no modules are assigned to the Sidebar positions, the Mainbody will become full width.
To find out about Gantry Layouts and Module Widths, check out this screencast which covers basic concepts of configuring layout with a combination of module setting and the Gantry layout control.
Check out this quick screencast on Module Positions to get an overview of how module positions work within Gantry Framework. Click below button to learn how to add a new row of module positions.