The PHP ftp_size() function returns the size of a specified file on the FTP server.

<?php
// connect and login to FTP server
$ftp_server = 'ftp.example.com';

$ftp_conn = ftp_connect($ftp_server) or die('Could not connect to ' . $ftp_server);
$login = ftp_login($ftp_conn, $ftp_username, $ftp_userpass);

$file = 'server-file.txt';

// get file size
$fsize = ftp_size($ftp_conn, $file);

if ($fsize != -1) {
    echo "$file is $fsize bytes";
}
else {
    echo 'Error getting the file size';
}

// close connection
ftp_close($ftp_conn);
?>

// gets the size of a file on the FTP server

ftp_size(ftp_conn, file)