There is an option to increase or decrease the default width of the content area. To change the value just slide the pointer to the left or right or enter the fixed value in the box.
Go to Appearance >> Customize >> General Settings >> Container Width
Container Width
Set the width of the container.
Content Width
Set the width of the content which is covered on the screen.
Sidebar Width
Set the width of the sidebar which is covered on the screen.
