Plesk runs on a separate Apache instance which uses its own configuration. In order to change the maximum file size, edit the file /usr/local/psa/admin/conf/php.ini.
Add or update the values for upload_max_filesize and post_max_size, e.g. 25M for 25 megabytes.
As last step, restart the Apache instance responsible for Plesk user interface.
/etc/init.d/psa stop /etc/init.d/psa.start