Canvas and aspect ratio

With nothing selected, the top of the Background tab has CANVAS.

The presets

Original, 16:9, 9:16, 1:1, 4:3, 4:5, 16:10 and 10:16.

Every fixed preset is 1080 on the short side. 16:9 is 1920 by 1080, 9:16 is 1080 by 1920, and so on.

What Original means

The shape of the first recording in the project. Record a 1920 by 1080 area and Original is 16:9. Record one of the portrait presets and Original is already portrait.

In a project with two takes of different shapes, Original follows the first one, and the second is fitted inside it.

What happens to the recording

The recording keeps its own shape. It is not stretched, ever. What changes is the canvas around it, and the recording is placed inside that canvas at the size the padding allows.

Going from 16:9 to 9:16 therefore leaves a lot of empty canvas above and below your recording. You have three ways to use it: a background, the live backdrop made from the picture itself, or the camera stacked above the screen.

Which to choose

16:9 for YouTube and for anything embedded on a web page.

9:16 for Reels, Shorts and TikTok.

1:1 and 4:5 for a feed post, where 4:5 takes up more of a phone screen than a square does.

Change it whenever you like. It is one undo away, and everything on the timeline survives the change.