gistfile1.html

<script>
/**
 * Version 1.1
 * Makes Vimeo and YouTube videos on Tumblr larger than the max of 500px that Tumblr forces
 * Change the 650 to whatever you want, but do NOT add px or %
 * The height, by default, is set to auto which will calculate the 16:9 video size.
 * You can set the height to a static number as well if you want.
 * If you have any questions feel free to ask me: @oscargodson | oscargodson.com
 */
(function(){
  var iframes = document.body.getElementsByTagName('iframe')
  ,   newVimeoWidth = 650
  ,   newVimeoHeight = 'auto';
  for(x in iframes){
    if(iframes[x].src && iframes[x].src.indexOf('player.vimeo.com') > -1 ||
       iframes[x].src && iframes[x].src.indexOf('youtube.com') > -1){
      if(newVimeoHeight == 'auto'){
        newVimeoHeight = ( newVimeoWidth * 9 ) / 16;
      }
      iframes[x].width = newVimeoWidth;
      iframes[x].height = newVimeoHeight
    }
  }
})();
</script>
</body>