Size presets

Under the three capture modes on the same menu is a list of sizes. Choosing one sets the capture area to that exact size, centred on your main screen, and switches to area mode.

Landscape: 1280 by 720, 1920 by 1080, 4:3, 16:10 and Square.

Portrait: 720 by 1280, 900 by 1600, 1080 by 1920 and 1440 by 2560.

A preset that would not fit on your screen is scaled down until it does, keeping its shape. On a 1080p screen the 1440 by 2560 preset lands as a smaller rectangle of exactly the same proportions, and the video is still the right shape for where it is going.

Why record at the size you will publish

You can always change the shape later in the editor, on the Background tab, and the editor will do a decent job of it. But a recording made at the shape it will be published at needs no cropping and no blurred filler at the sides, and every pixel you captured is a pixel the viewer sees. If you know the video is for Reels, start with a portrait preset.

The camera band on a portrait frame

At the bottom of the same menu is Camera above the screen on a portrait frame. With it on, a portrait capture area is split: the upper part is left for your camera and only the lower part is recorded from the screen. That is the layout most phone tutorials use, and it means the camera is not sitting on top of the thing you are demonstrating.

It does nothing on a landscape frame.

Custom

Custom at the foot of the list is the same as picking Region: drag your own box.