I have used this before in functions.php to display the file size in an easily readable format:
$bytes = filesize($file);
$s = array('b', 'Kb', 'Mb', 'Gb');
$e = floor(log($bytes)/log(1024));
return sprintf('%.2f '.$s[$e], ($bytes/pow(1024, floor($e))));
And then in my template:
echo getSize('insert reference to file here');
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