Error 414 when posting/updating via API

Hello everybody!
When trying to post or update a message whose content exceeds a certain size (the exact size I cannot detemine yet - is there a way to check this?) NGINX refuses to accept the POST or PUT requests and returns Error 414.
Is there a way to change (permanently) the default configuration delivered with the Discourse image, so that a larger payload size (at least matching the default 32000 characters in the admin settings) is accepted via API?