I get the following variables from the database:

$max - the number of times the most popular tag has been used.
$min - the number of times the least popular tag has been used.
$distribution - ($max - $min) / 5;

Then I just go through each tag from the database and select the CSS class for each one…

Let’s say that $tagcount is the amount of times a specific tag has been used…

if($tagcount == $min){
$class = “link13″; /* smallest */
}elseif($tagcount == $max){
$class = “link28″; /* biggest */
}elseif($tagcount > ($min + ($distribution*2))){
$class = “link26″; /* big */
}elseif ($tagcount > ($min + $distribution)){
$class = “link20″; /* medium */
}else{
$class = “link15″; /* small */
}

It’s a bit quick and dirty but it works (so far)