The maximum size of a file that can be uploaded to the server using the FIle Manager depends on Plesk PHP settings.

To set the maximum size of a file that can be uploaded:

1. Open the /usr/local/psa/admin/conf/php.ini file for editing.
2. Set the value of the post_max_size and upload_max_filesize directives to the desired maximum file upload size.
3. Restart the web server by running the /etc/init.d/psa restart command.

Note that the changes are server-wide and will affect both the provider and all customers.