I had a project that required posting a 2.5 million character QueryString to a web page. The server itself only parsed URI's that were 5,400 characters or less. After trying several different sets of code for WebRequest/WebResponse, WebClient, and Sockets, I finally found the following code that solved my problem:

HttpWebRequest webReq;
HttpWebResponse webResp = null;
string Response = "";
Stream reqStream = null;
webReq = (HttpWebRequest)WebRequest.Create(strURL);

Byte[] bytes = Encoding.UTF8.GetBytes("xml_doc=" + HttpUtility.UrlEncode(strQueryString));
webReq.ContentType = "application/x-www-form-urlencoded";
webReq.Method = "POST";
webReq.ContentLength = bytes.Length;
reqStream = webReq.GetRequestStream();
reqStream.Write(bytes, 0, bytes.Length);
reqStream.Close();
webResp = (HttpWebResponse)webReq.GetResponse();

if (webResp.StatusCode == HttpStatusCode.OK)
{
     StreamReader loResponseStream = new StreamReader(webResp.GetResponseStream(), Encoding.UTF8);
     Response = loResponseStream.ReadToEnd();
}

webResp.Close();
reqStream = null;
webResp = null;
webReq = null;