Configure at Admin → Extensions → Template Manager → rt_metropolis, then go to Layouts to set the grid widths and allocated positions.
Injected Gantry Features
There are Features that are injected into a position when enabled, and are stacked vertically; which includes: Social Buttons, Date, Font Sizer, Login Panel, Popup Module, Branding, Copyright, To-Top Scroller, System Messages, Reset Settings, and Google Analytics.
MainBody / Sidebar Layouts
Configure at Admin → Extensions → Template Manager → rt_metropolis, 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.