Make specific widget full width

Widgets will always be as wide as the column they populate in the layout. The only way to change a widget width would be to select a layout that allows for what you want. For example, the highlighted layout below would allow one or more wide widgets above a column with a right sidebar. Please note that the layout is specific to the area you are editing (Header/Body/Footer) it does not define the entire page. 

Another option would be to use css to override the width in a similar way to the group banner or featured content widgets which can go full browser width