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
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' );