Not sure I understand you correctly. The only way to change it to full width over there is to select ‘No Sidebar, full width”. I did try that even though we want to keep our sidebar, but it’s still not full width, the posts below it stick out on both sides.
We have reduce the width size on single page as of user point of view to read the content.
You can add below line of code in Appearance > Customize > Additional CSS to make the content area to full width.