Not 100 % sure if this is the right place to change it because I didn't test it but you can try it

Find in Display.php:
<td class="$css" bgcolor="$windowbg" valign="top" width="15%" rowspan="2">
$usernamelink<br><font size="1">$title
$memberinf<br>
EOT;
if($muserID != "-1") {
//Messy crap - perhaps someone can alter this later
$ptext=$userprofile[$mname]['personalText'];
$ptext = str_replace (""", "\"", $ptext);
$ptext = str_replace ("'", "'", $ptext);
$ptext = str_replace ("&", "&", $ptext);
$ptext = str_replace ("<", "<", $ptext);
$ptext = str_replace (">", ">", $ptext);
print " $star<br><BR>\n$online ".$userprofile[$mname]['gender'];
if (($sa[0] == 1) OR ($username == "Guest")) { print "\n $postinfo\n ".$userprofile[$mname]['avatar'].$ptext; }
elseif ($sa[0] == 0) { print "\n $postinfo\n ".$ptext; }
print "\n {$userprofile[$mname]['ICQ']} $icq $msn $yim {$userprofile[$mname]['AIM']}<BR>\n";
}
if($muserID == '-1') {
print " <BR><a href=\"mailto:$memail\">$img[email_sm]</a>\n";
}else if ($userprofile[$mname]['hideEmail'] != "1" || $settings[7] == "Administrator" || $allow_hide_email != '1') {
print " $profbutton".$userprofile[$mname]['websiteUrl']." <a href=\"mailto:$memail\">$img[email_sm]</a>$sendm\n";
} else {
print" $profbutton{$userprofile[$mname]['websiteUrl']}$sendm \n";
}
print <<< EOT
</font>
</td>
<td class="$css" bgcolor="$windowbg" valign="top" width="85%" height="100%">
There you can notice two cells, first has the width of 15 % and the second is 85 %. By changing these you can control the member info column and the post column.

Try changing them to 20/80 for example.