2M @ WP

root@yourhostname:~# cat /etc/apache2/sites-available/yoursite.conf
[...]
# WP Based root
DocumentRoot /var/www/html/wordpress
# To enable custom php.ini
PHPINIDir /var/www/html/wordpress
[...]

root@yourhostname:~# cat /var/www/html/wordpress/php.ini
upload_max_filesize = 32M
post_max_size = 32M