Get File Size On An FTP in C#

Try reqSize.Method = WebRequestMethods.Ftp.GetFileSize; instead of GetDateTimestamp

This worked for me:

FtpWebRequest request = (FtpWebRequest)FtpWebRequest.Create(new Uri("ftp://servername/filepath"));
request.Proxy = null;
request.Credentials = new NetworkCredential("user", "password");
request.Method = WebRequestMethods.Ftp.GetFileSize;

FtpWebResponse response = (FtpWebResponse)request.GetResponse();
long size = response.ContentLength;
response.Close();