413 Request Entity Too Large

The error / response code itself states that it arised because of large amount of form submit data. For security reasons the server is configured to accept only some amount of data.

If your server is Apache and mod_security module is installed, then mod_security configurations need to be altered.
Open up mod_security.conf (usually located at /etc/httpd/conf.d/mod_security.conf)
Get to the lines containing SecRequestBodyNoFilesLimit and SecRequestBodyInMemoryLimit.
Update the settings with below values respectively.

SecRequestBodyNoFilesLimit 10486000 
SecRequestBodyInMemoryLimit 10486000

That's it. Change 10486000 according to your needs. You can observe the request headers while submitting the form to know how much value would be enough.