@namespace url(http://www.w3.org/1999/xhtml);
@-moz-document url-prefix("http://youtube.com/watch"),
url-prefix("http://uk.youtube.com/watch"), 
url-prefix("http://pl.youtube.com/watch") {

/* change the url-prefix to match your language Youtube domain */

#movie_player {
  width: 875px !important;
  height: 700px !important; /* change to your needs! */
}

#otherVidsDiv {
  margin-top: 710px !important; /* height + 10px! */
}

}