There is a limit to the ASP request object that means that it can not hold more than 100kb of data.
As the request object also holds other data as well as the post form data such as cookies, browser details, page refering, etc. The amount of characters that ASP can handle in a post depends on other outside influences, such as if other cookies have been set by other pages on the site and how large these cookies are, the type and version of browser the person is using, the refering page, the length of the URL to the file, etc., etc.