My nginx-ingress-controller is in the ingress-nginx namespace and I've set the large-client-header-buffers to 4 16k, 4 32k etc.

kind: ConfigMap
apiVersion: v1
metadata:
  name: nginx-configuration
  namespace: ingress-nginx  
data:
  proxy-buffer-size: "16k"
  large-client-header-buffers: "4 16k"

When I inspect the configuration in the nginx-controller pod I see:

 kubectl exec -n ingress-nginx nginx-ingress-controller-65fd579494-jptxh cat /etc/nginx/nginx.conf | grep large_client_header
        large_client_header_buffers     4 16k;

So everything seems to be configured correctly, still I get the error message 400 Bad Request Request Header Or Cookie Too Large