在c#中使用FTP下载图像文件，图像文件大小由固定大小数组限制

我正在尝试使用ftp下载一个图像文件，在每个代码上，我已经查找了一个fixed sized array正在使用(因为bufferSize正在下面的代码中使用)，我如何才能有效地解决这个问题，并在运行时调整缓冲区的大小，因为我正在处理显著大小的图像。

 string[] ftpInfo = (string[])e.Argument;
            string uri = String.Format("ftp://{0}/{1}/images/{2}", ftpInfo[1], ftpInfo[2], ftpInfo[5]);
            FtpWebRequest request = (FtpWebRequest)WebRequest.Create(uri);
            request.Method = WebRequestMethods.Ftp.DownloadFile;
            request.UseBinary = true;
            request.Credentials = new NetworkCredential(ftpInfo[3], ftpInfo[4]);
            FtpWebResponse response = (FtpWebResponse)request.GetResponse();
            Stream ftpStream = response.GetResponseStream();
            long cl = response.ContentLength;
            int bufferSize = 4096;  //Image file cannot be greater than 40 Kb
            int readCount = 0;
            byte[] buffer = new byte[bufferSize];
            MemoryStream memStream = new MemoryStream();
            readCount = ftpStream.Read(buffer, 0, bufferSize);
            while (readCount > 0)
            {
                memStream.Write(buffer, 0, readCount);
                readCount = ftpStream.Read(buffer, 0, bufferSize);
            }
            response.Close();

ftpStream.Read(buffer, 0, bufferSize);