Use with stylish browser extension.

room11-sidebar.css

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document url("http://chat.stackoverflow.com/rooms/11/php") {
  #sidebar {
      bottom: 88px;
      height: auto;
      overflow: auto;
      padding-left:34px;
  }
 
  #room-tags,
  #roomtitle,
  #roomdesc,
  #room-ad {
    display:none;
  }
  
  #present-users {
    height:52px;
    overflow: hidden;
  }
  
  .present-user {
    height:26px;
    font-size:initial;
  }
  
  .present-user img {
    width:26px;
    height:26px;
  }
}