This is a premium feature available with Astra Pro Addon plugin. To use these Pro features, you need to have the Astra theme along with the Astra Pro Addon installed on your website.
Please refer the following document, to begin with, Site Layout.
A website generally has the main wrapper followed by the container.
The main wrapper consists of header, content, sidebar, and footer — the entire website;And the container is the child wrapper that includes a content & sidebar.
In Max Width Site Layout, the main wrapper of the website is set to a certain maximum width. We have also provided settings for you to control the top and bottom margin, so you can design your website to appear like a real box.
You can find the setting to manage Max Width Site Layout under Appearance > Customize > Global > Container > Site Layout
Other Notes:If you need to set a background image for the entire site please check this document: Global Colors – Astra Theme.