Line length in Editor

Just installed Scrivener 3. How do I widen the length of the line in the Editor?

Please do the built-in interactive tutorial, even if you are familiar with version 2. There are some significant differences.

Editor width: Preferences -> Appearance -> Main Editor, and there you have a tick box “Use fixed width editor” and can specify if you want that or if you want the text to fill the Main Editor.