Can you explain a bit more as to why / how you want it as a background video?
For example: You can increase the size of the imported video by dragging it's handles and then send it to the back of the element order (Arrange->Send to Back). Also you can turn off the controls by unchecking the controls box in the Element inspector.
If you want it to stretch the whole window length then you must switch on "scale" in the Scene inspector and click the arrows
in the Metrics inspector.
There are other ways where you can use a script like BigVideo.js
in order to set the video as a backdrop.