What are layout settings in the Themes Editor?
Layout settings help to customize how a published module appears in terms of the width and density of the user interface (UI) elements.
Admins can choose between a compact theme and a standard theme. The compact theme has less spacing between the different user UI elements, i.e., space between the content field and options in input nodes.
640px will be the minimum width and also default setting for themes. The width can get increased up to 4000px. This ensures that even on large screens, the theme can be shown on the full screen width. Generally, if the screen is wider than the chosen width in the theme settings, it will be positioned in the center. In case the screen is smaller than the chosen width in the theme settings, the theme will default to full screen width (100%). This also implies mobile devices.