try

#wpforo #wpforo-wrap .wpforo-post .wpf-right .wpforo-post-content .wp-video .mejs-container-fullscreen .wp-video-shortcode{
  height: 100% !important;
  width: 100% !important;
}

It works, thanks a lot Tutrix. 👍 👍 👍 👍 

try

#wpforo #wpforo-wrap .wpforo-post .wpf-right .wpforo-post-content .wp-video .mejs-container-fullscreen .wp-video-shortcode{
  height: 100% !important;
  width: 100% !important;
}

Remember my old thread?

https://wpforo.com/community/how-to-and-troubleshooting-2/how-do-i-change-the-size-of-embedded-videos-with-code-video-src/

Videos don't play properly when you press full screen, any idea?Thanks. 😥