There's a proxy-body-size option that can be configured via an annotation on ingress objects.  This option maps to client_max_body_size in the nginx configuration.

---
apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: some-ingress
  annotations:
    kubernetes.io/ingress.class: nginx
	# Change 5m to whatever size you want
    nginx.ingress.kubernetes.io/proxy-body-size: 5m
spec:
...