Max length of send() data param on XMLHttpRequest Post

Is there a documented max to the length of the string data you can use in the send method of an XMLHttpRequest for the major browser implementations?

I am running into an issue with a JavaScript XMLHttpRequest Post failing in FireFox 3 when the data is over approx 3k. I was assuming the Post would behave the same as a conventional Form Post.

The W3C docs mention the data param of the send method is a DOMString but I am not sure how the major browsers implement that.

Here is a simplified version of my JavaScript, if bigText is over about 3k it fails, otherwise it works…

var xhReq = createXMLHttpRequest();

function createXMLHttpRequest() {
  try { return new ActiveXObject("Msxml2.XMLHTTP"); } catch (e) {}
  try { return new ActiveXObject("Microsoft.XMLHTTP"); } catch (e) {}
  try { return new XMLHttpRequest(); } catch(e) {}
  alert("XMLHttpRequest not supported");
  return null;

function mySubmit(id, bigText) {
  var url    = "SubmitPost.cfm";
  var params = "id=" + id + "&bigtext=" + encodeURI(bigText);"POST", url, true);

  //Send the header information along with the request
  xhReq.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  xhReq.setRequestHeader("Content-length", params.length);
  xhReq.setRequestHeader("Connection", "close");

  xhReq.onreadystatechange = onPostSubmit;

function onPostSubmit() {

  if (xhReq.readyState==4 || xhReq.readyState=="complete")
     if (xhReq.status != 200)


I believe the maximum length depends not only on the browser, but also on the web server. For example, the Apache HTTP server has a LimitRequestBody directive which allows anywhere from 0 bytes to 2GB worth of data.