Flash Player 6.
my_video.width
Read-only property; an integer specifying the width of the video stream, in pixels. This value is the same as the Camera.width
property of the Camera object that is capturing the video stream. You may want to use this property, for example, to ensure that the user is seeing the video at the same size at which it was captured, regardless of the actual size of the Video object on the Stage.
See the example for Video.height.