'maximal_form_length' specifies how many datas the HTTP client is allowed to send in a POST request.
The default value is 2^16 (64 KB).
You may want to increase this value if the client needs to send really long forms result or sets of options, but increasing the number will make your HTTP server weaker because many clients sending big POST requests will currently exhaust the server resources.

An example:

pliant module /pliant/protocol/http/server.pli command 'http_server port 8080 maximal_form_length 2^20 name "MYSERVER"'