Appearance > Customize > Site Width
The default width of the page content is 1200px when viewed on screens wider than 1120px. For all screens smaller than 1120px wide, the width automatically resizes to the width of the screen or device that the website is being viewed on.
You can change the default width for screens wider than 1120px. Simply use the up/down arrows to change the width to your preferred setting or tick the checkbox next to ‘Full Width’ to make the page content 100% of the screen width.