The following are three key information items on SharePoint Online widths:
- SharePoint Online uses a responsive grid system based on the Office UI Fabric The basic grid in the SharePoint desktop experiences is a 12-column structure.
- In a communication site the max width listed as being 1204 px.
- In a team site the max width is listed as being 1204 px.
Note: these aspects may vary based on your screen size, system and layout of the page.
