First of all I hope this form is submitted by the POST method and not GET, because GET has severe limits on data size. Unfortunately, your web server or a proxy intermediary may be cutting off the request body even if you are doing POST. So it depends entirely on the server side APIs you are using to process your form data. It is not a property of HTML or HTTP.

Update: For example, Microsoft's ASP limits the size of a form submission using the application/x-www-form-urlencoded encoding to 100KB.