You need to edit the correct php.ini file and increase the value of the following variables to the desired size:
memory_limit, upload_max_filesize and post_max_size
The php.ini file is located at:
On Linux server:
/usr/local/psa/admin/conf/php.ini
On Windows server:
C:\Program Files (x86)\Parallels\Plesk\admin\php.ini
Then you will need to restart your webserver & PSA.