In the 4.2 release (in April), the description length will be a configurable parameter. For now though, if you need to change it, edit the /plugins/content/jfbccontent/jfbccontent.php file. Search for "100" and you'll find a line like below:
$articleText = substr($articleText, 0, 100);
Simply replace the 100 with whatever length you'd prefer to use.
Hope that helps, and good luck!
Alex