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