Hey Pilgrim
It sounds like the post/reply screens don't recognize that setting. You can make specific width changes in the newpost/newreply templates around this area:
<tr>
<td class="alt-2" valign="top">
<label for="texteditor">{$lang.POST_TEXT}</label>
</td>
<td class="alt-1" valign="top">
{$text_editor}
</td>
</tr>