open forum/forum.post.php and edit
PHP Code:
$maxsubjectlength = 40;
then use phpmyadmin and increase the lenght of subject field in xbtit_topics table to 80 or what ya need max 250
you will also need to edit style/xbtit_default/forum.post.tpl
find and edit 40 to same values you choose above
HTML Code:
<td class="header"><tag:language.SUBJECT /></td>
<td align="left" style="padding: 0px" class="header"><input type="text" size="60" maxlength="40" name="subject" value="<tag:post_subject />" /></td>