[areostar]

if(stage.displayState == StageDisplayState.NORMAL)
{
stage.fullScreenSourceRect = new Rectangle(_mediaPlayer.x,_mediaPlayer.y, _mediaPlayer.width , _mediaPlayer.height); 
stage.displayState = StageDisplayState.FULL_SCREEN;
}
else
{
   stage.displayState = StageDisplayState.NORMAL;
}

К примеру так