Dejalo así:
{$ignore_bit}
<table border="0" cellspacing="{$theme['borderwidth']}" cellpadding="{$theme['tablespace']}" class="tborder" style="{$post_extra_style} {$post_visibility}" id="post_{$post['pid']}">
<tr>
<td class="{$altbg}" width="150px" valign="top" style="white-space: nowrap; text-align: center;"><a name="pid{$post['pid']}" id="pid{$post['pid']}"></a>
<div class="pbbox">
<strong><span class="largetext">{$post['profilelink']}</span></strong> {$post['onlinestatus']}<br />
<span class="smalltext">
{$post['usertitle']}<br />
{$post['userstars']}
{$post['groupimage']}
{$post['useravatar']}<br />
<br/>
<center>
<div class="postbit2"><span class="pbh">Pais:</span> <img src="images/flags/{$post['fid1']}.gif" border="0" alt="{$post['fid1']}" /></div>
<div class="postbit2"><span class="pbh">Sexo:</span> <img src="images/{$post['fid3']}.gif" border="0" alt="{$post['fid3']}" /></div>
<div class="postbit2"><span class="pbh">Ubicacion:</span>{$post['fid4']}</div>
{$post['user_details']}{$post['newpoints_postbit']}
<div class="postbit2"><span class="pbh"><a href="{$mybb->settings['bburl']}/plaza.php"><img src="images/moneda.png"></a>: <span id="postmoney_{$post['pid']}">{$post[MYPLAZA_MONEY_COLUMN]}<span class="pbh"></span></span></span><br /></span>
<hr>
<b>items:</b>{$post['invitems']}
</div>
</div>
</td>
<td class="{$altbg}" valign="top">
<table width="100%">
<tr><td>{$post['posturl']}<span class="smalltext"><strong>{$post['icon']}{$post['subject']} {$post['subject_extra']}</strong></span>
</center>
<br />
<div id="pid_{$post['pid']}" style="padding: 5px 0 5px 0;">
{$post['message']}
</div>
{$post['attachments']}
{$post['signature']}
<div style="text-align: right; vertical-align: bottom;" id="post_meta_{$post['pid']}">
<div id="edited_by_{$post['pid']}">{$post['editedmsg']}</div>
</div>
</td></tr>
</table>
</td>
</tr>
<tr>
<td class="{$altbg}" style="white-space: nowrap; text-align: center; vertical-align: middle;"><span class="smalltext">{$post['postdate']} {$post['posttime']}</span></td>
<td class="{$altbg}" style="vertical-align: middle;">
<table width="100%" border="0" cellpadding="0" cellspacing="0">
<tr valign="bottom">
<td align="left" ><span class="smalltext">{$post['button_email']}{$post['button_pm']}{$post['button_find']}</span></td>
<td align="right">{$post['button_edit']}{$post['button_quote']}{$post['button_multiquote']}{$post['button_reply_pm']}{$post['button_replyall_pm']}{$post['button_forward_pm']}{$post['button_delete_pm']}</td>
</tr>
</table>
</td>
</tr>
</table>
Y luego le añades un ancho al pbox desde el CSS.