<?php

$file = 'somefile.txt';

// set up basic connection
$ftp = ftp_connect($ftp_server);

// login with username and password
$login_result = ftp_login($ftp, $ftp_user_name, $ftp_user_pass);

// get the size of $file
$res = ftp_size($ftp, $file);

if ($res != -1) {
    echo "size of $file is $res bytes";
} else {
    echo "couldn't get the size";
}

// close the connection
ftp_close($ftp);

?>