Max Post Size means how much the size of a POST body data can be.
The post_max_size can be set by modifying multiple files. They are .htaccess, php.ini, .user.ini, or wp-config.php. files. You are gonna need FTP access to modify those files. You can find the credentials for your FTP access from your hosting provider
To set post_max_size please follow the steps given below,
1.Go to your WordPress root installation folder and locate any of these files .htaccess, php.ini, .user.ini or wp-config.php
2.Select any of the files above and Edit and paste the code given below,
3.Save the file.
php_value post_max_size 32M
@ini_set( 'post_max_size' , '32M' );
The recommended size is ’32M’ minimum.