UE4 Get Viewport / Screen Size and Center

gistfile1.txt

/*
IMPORTANT! 
In order to be able to use this functions, in the file "YourProject.h" change this:

    #include "EngineMinimal.h"
    
For this:

    #include "Engine.h"

*/

//Viewport Size
const FVector2D ViewportSize = FVector2D(GEngine->GameViewport->Viewport->GetSizeXY());
 
//Viewport Center!            
const FVector2D  ViewportCenter =  FVector2D(ViewportSize.X/2, ViewportSize.Y/2);

Another way to do it is like this:
const FVector2D ViewportSize = FVector2D(GEngine->GameUserSettings->GetLastConfirmedScreenResolution());

Thank you!