Nginx response 414 URI Too Long even with very large large_client_header_buffers setting

Question

I am using nginx version: nginx/1.10.1 and proxy to Rails app. I need to accept a very long URL and I update it with

client_header_buffer_size 512k;
large_client_header_buffers 4 512k;

But this only works for URL length around 8'000 characters. URL with more than that will not work.

I tried to put these header settings into http {} and server {} block, but does not work.

Did I miss anything? It should work according to Nginx's doc http://nginx.org/en/docs/http/ngx_http_core_module.html#client_header_buffer_size