Default you only can upload file 2MB. To change this, you have to set the value of upload_max_filesize and post_max_size in php.ini
nginx run php-fpm, so please edit file: /etc/php5/fpm/php.ini
You have to restart both nginx and php-fpm
/etc/init.d/nginx restart
/etc/init.d/php5-fpm restart
