this will show you the size in Megabytes, if you want Kilobytes instead change
number_format(($total[size]/1024/1024),2, '.', ''). 'MB';
to:
number_format(($total[size]/1024),2, '.', ''). 'KB';
untested, but try it out:
$result = mysql_query("SELECT SUM(LENGTH(column)) AS size FROM table");
$total = mysql_fetch_array($result);
echo number_format(($total[size]/1024/1024),2, '.', ''). 'Mb';