Format your Filesize in WordPress

functions.php

// size calculation
$docsize = @filesize( get_attached_file ( $attachment->ID ) );
// show the attachment, 
echo $filename . ' — ' . $docsize;

// echoes something like attachment.zip — 13143

// use size_format() instead. start with size calculation
$docsize = @filesize( get_attached_file( $attachment->ID ) );
if (FALSE === $docsize)
    $docsize = 0;
else
    $docsize = size_format($docsize, 2);
// show the attachment, 
echo $filename . ' — ' . $docsize;

// attachment.zip — 13.14 kB