[TECK]

i think the best way is to use the strlen() to limit the lenght.
for example...
in functions.php, replace:

      $nav_title=$threadinfo[title];

with:

      $nav_title=$threadinfo[title];
      if (strlen($threadinfo[title]) > 25) {
        $nav_title = substr($threadinfo[title], 0, 23) . '...';
      }

now all you have to do is change the the values 25 and 23 to whatever you like your title to be in lenght. value 25 represents the total title lenght and 23 represents the total title lenght - 2 characters, because you add the "..." dots at the end.