This feature is on the Settings screen in the PublishPress Blocks plugin. This feature can modify the width of the Gutenberg editor screen.

You can also access this setting by clicking the PublishPress Blocks icon when you're writing a post:

Screen Shot 2020 12 09 At 17.40.55

This image shows the “Original” setting with a PublishPress Image block. When using “Default”, you'll see the default width defined by the theme.

This final image shows the same screen with the “Full width” option: