The login screen on Ubuntu fails to go into full screen or to fill window.
However, when logging-in to user accounts, the screen does go full.
I have already installed Guest Additions correctly and i use the Auto-resize Guest Display as shown at the image below.
That is maybe happening because of the console resolution (800x600).
But i don't want to make it static, but to make it resizable, by adjusting the host's window size.