Try editing the post_max_size variable, if you have access to your php.ini file. This variable is the limit of the entire body of the request, which could include multiple files (source – stackoverflow).
If you don’t have access to it you can try editing your functions.php file and add:
@ini_set( 'post_max_size', '64M'); //this will set 64mb as limit
If that don’t work you can try editing your .htaccess file:
php_value post_max_size 64M
If that doesn’t work either, you can try creating a php.ini file in your root directory and add:
post_max_size = 64M
If that fails either you must contact your hosting company and ask them to increase the size of it.
p.s. If you have access to cPanel check out the tools there. Some hosting companies have options to edit these variables.