opendir(DIR, $user{site_id}) 
   || die "Unable to open directory $user{site_id}: $!";
my @userfiles = sort grep !/^\.\.?\z/, readdir DIR;
close DIR;  

foreach my $file (@userfiles) {
    my $size = format_size(-s "$user{site_id}/$file");
    print $size, "\n";
}

sub format_size {
    my $bytes = shift;
    if ($bytes >= 1048576) {
        return sprintf("%.2f MB", $bytes / 1048576);
    } elsif ($bytes >= 1024) {
        return sprintf("%.2f KB", $bytes / 1024);
    } else {
        return "$bytes Bytes";
    }
}