413 errors occur when the request body is larger than the server is configured to allow. Here’s how you can fix it, depending on your web server:
Apache: Set the LimitRequestBody directive in either your httpd.conf file or a local .htaccess file. (https://stackoverflow.com/a/3719358/1688568)
Nginx: Set the client_max_body_size directive in nginx.conf (http://www.cyberciti.biz/faq/linux-unix-bsd-nginx-413-request-entity-too-large/)
IIS: Set the uploadreadaheadsize config setting (http://blogs.msdn.com/b/jiruss/archive/2007/04/13/http-413-request-entity-too-large-can-t-upload-large-files-using-iis6.aspx)
You also might want to try increasing PHP’s upload_max_filesize and post_max_size settings in php.ini or using the
php_value directive in your .htaccess file if you’re using Apache (http://davidwalsh.name/php-values-htaccess).