global $db_prefix, $scripturl;
$query = "SELECT t.tag AS tag, l.ID_TAG, COUNT(l.ID_TAG) AS quantity
  FROM {$db_prefix}tags as t, {$db_prefix}tags_log as l WHERE t.ID_TAG = l.ID_TAG
  GROUP BY l.ID_TAG
  ORDER BY l.ID DESC LIMIT 50";

$result = db_query($query, __FILE__, __LINE__);

$tags = array();

$tags2 = array();

while ($row = mysql_fetch_array($result))
{
    $tags[$row['tag']] = $row['quantity'];
    $tags2[$row['tag']] = $row['ID_TAG'];
}

if(count($tags2) > 0)
{
// change these font sizes if you will
$max_size = 250; // max font size in %
$min_size = 100; // min font size in %

// get the largest and smallest array values
$max_qty = max(array_values($tags));
$min_qty = min(array_values($tags));

// find the range of values
$spread = $max_qty - $min_qty;
if (0 == $spread)
{ // we don't want to divide by zero
    $spread = 1;
}

// determine the font-size increment
// this is the increase per tag quantity (times used)
$step = ($max_size - $min_size)/($spread);

// loop through our tag array
$poptags = '';
$row_count = 0;
foreach ($tags as $key => $value)
{
$row_count++;
    // calculate CSS font-size
    // find the $value in excess of $min_qty
    // multiply by the font-size increment ($size)
    // and add the $min_size set above
    $size = $min_size + (($value - $min_qty) * $step);
    // uncomment if you want sizes in whole %:
    // $size = ceil($size);

    // you'll need to put the link destination in place of the #
    // (assuming your tag links to some sort of details page)
    $poptags .= '<a href="' . $scripturl . '?action=tags;id=' . $tags2[$key] . '" style="font-size: '.$size.'%"';
    // perhaps adjust this title attribute for the things that are tagged
   $poptags .= ' title="'.$value.' things tagged with '.$key.'"';
   $poptags .= '>'.$key.'</a> ';
   if ($row_count > 5)
   {
    $poptags .= '<br />';
    $row_count =0;
   }
}
}
echo $poptags;