Update PHP.ini

sudo vi /etc/php5/fpm/php.ini #or wherever your php.ini is

Set the value to

upload_max_filesize = 8M
post_max_size = 10M

This is just a common sense that post_max_size should always be larger than upload_max_filesize.

Change in Nginx config per site

http {
        client_max_body_size 10m;

Restart php and nginx

sudo service php5-fpm reload
sudo nginx -t && sudo service nginx reload