put in /etc/nginx/nginx.conf

http {
 ......
 client_max_body_size 200M;
 ......
}

and in /etc/nginx/sites-available/default

server {
 .....
 client_max_body_size 200M;
 ......
}

To see nginx errors, run:

nginx -t

if you are not sure, try : nginx -h, for help