Your screen design needs to match the shape of the display it runs on. In Version 8 you do not set that by hand. You pick the device your event will run on, and the screen editor matches its resolution and orientation for you.
Pick your device
Open the screen editor for your event.
Click Select a device.
Filter to Windows and choose the device your event will run on. The one you are on is marked Current device.
The editor switches to that display's resolution and screen orientation.
Tip: devices are listed by name. Rename yours in LumaBooth under About > Device Name so it is easy to spot.
Portrait and magic mirror setups
Choosing a portrait-mounted display sets the orientation along with it, so there is no separate step. You can still set Screen orientation yourself if you want a design that does not match the display.
If your device is not listed
Update LumaBooth first: Help > Check for Updates. Versions before 8.4.0 could show an empty device list on some setups, and that is fixed.
About aspect ratio
Aspect ratio is the ratio of width to height. 1920x1080 is 16:9, and a Surface Pro 9 is 16:10. If your design and your display are different shapes you will see a grey bar along one edge in the editor. Move your screen elements clear of it, or switch the design to your display's ratio.