In 4.11, after setting Standalone Window Size to 1920x1080, it plays in correct, desired HD resolution.
In 4.12, it ignores it when playing in ‘Standalone Game’ window (it opens is some other, smaller size). Only New Editor Window (PIE) respects the desired resolution.
in 4.23 still same “DX12 -fullscreen command” error:
First, it uses your desktop resolution in “fullscreen” mode, with the game relegated to a portion of the screen. If you have a 4K display and set the game to fullscreen 1080p, you end up with the game occupying the top-left quadrant of the display. The solution is to drop the desktop resolution to 1080p before launching the game. And to reiterate, this does not happen in DX11 mode.