I just tried to upload a 16×9 HD video clip to Facebook, only to discover that they actually rescale everything to some funky dimensions. The result was my video image was squeezed into some crazy aspect ratio. So I did some research and learned that your 16×9 video needs to be 400 x 225 pixels in order for them not to scale it. I know that size is hell of ridiculously small, but they have millions of videos to host, so I guess I can accept it. Anyway, 400 x 225 is your man.