Explanation:
Graphical user interface, text, application Description automatically generated

Box 1: Tablet form factor
Box 2: Aspect version
Customize a canvas app by changing its screen size.
* Sign in to Power Apps.
* Open the app to edit.
* Select Settings.
* Select Display.
* (Tablet apps only) Under Aspect ratio, perform either of these steps:
* Select the ratio that matches the target device for this app.
* Select Custom to set your own size, and then specify a width between 50 - 3840 and a height between 50
- 2160.

Reference:
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/set-aspect-ratio-portrait-landscape