You can modify PHP post_max_size as per your needs using CloudLinux PHP Selector in Plesk.
1. Log in to your Plesk account.
2. Click on the Pull icon in the right sidebar and choose PHP Selector.
3. Click on the Options menu.
Note: If the Options page shows an error related to the native PHP version, change it to a non-native version first.
4. Look for post_max_size and click on the drop-down option displayed in front of it. Then choose the appropriate limit as per your requirements.