You need to pick one of virtual or percentage for your app. There is no third option.
The problem is getting the actual dimensions of the device you are running on before deciding what your display size is.
Assuming they work correctly (I haven't tested them), you should be able to call the GetDeviceHeight and GetDeviceWidth functions. Then you could make some decisions about your display.
But, you don't have that option for the initial images displayed for things like AGKSplash.png (which is now deprecated) and for the Default(-landscape/-portrait).png images for the iOS and Mac world.
Cheers,
Ancient Lady
AGK Community Tester