Depends on the ratio of video, but I think you can use something like this:
videoDisplay.scaleMode = ScaleMode.STRETCH;
or
videoDisplay.scaleMode = ScaleMode.LETTERBOX;
Update :
So in this case I think setting maintainAspectRatio
as false within your <mx:videoDisplay />
will help you, Although the main purpose of that is maintaining the original aspect ratio while resizing the video.