Hi @seeky,

Please navigate to Dashboard > Forums > Settings > Styles admin page, put the following CSS code in "Custom CSS Code" textarea, save it, delete all caches and check again.

@media screen and (max-width: 480px){
   h1#wpforo-title {
      font-size: 12px !important;
   }
}

Please change the red marked code before using.

Also please try on incognito mode. Because mobile browsers have a hard cache, you'll not be able to see the change immediately.