How do I change the size of the grey bar so it's the same as the post?
In Showthread templates -> showthread_classicheader change 15% in width attribute (in first table cell) to your desired width.
<tr>
<td class="tcat" width="15%"><span class="smalltext"><strong>{$lang->author}</strong></span></td>
<td class="tcat"><span class="smalltext"><strong>{$lang->message}</strong></span></td>
</tr>