How do I change the format of the scene (4/3, 16/9)?

When in  a session mode you can change the format of a scene, by default it's set to Auto (best fit) however you can choose 4/3 or 16/9 formats.

On a Scene window choose option Format from Menu bar:

Please note Format need to be changed before adding any backgrounds or layered assets otherwise your backgrounds will be reorganized and all layered assets- removed.