You can do it at your .htaccess file or at your php.ini file.
Add the following line to the .htaccess file. Replace x with the maximum HTTP POST file size that you want to set, in megabytes:
php_value post_max_size xM
If you have access to your php.ini, you can just add this directive (or change the existing value):
post_max_size = xM
Replace x with the maximum HTTP POST file size that you want to set.
Our plugin can show you this information at the page PHP INFO.