Good catch on the Jetty default. That requestHeaderSize limit comes from Jetty itself (the embedded web server Mango runs on), and Mango doesn't currently expose a mango.properties setting that maps to it. Mango exposes other web server tuning properties (web.threads.maximum, web.requests.queueSize, file upload and websocket buffer sizes, etc.), but request header size isn't wired through.
A couple of options:
Trim the proxy's injected headers. If your in-house proxy is adding several large headers, reducing what it injects (or moving some of that data to a request body/cookie instead) addresses the actual cause rather than working around a limit.
Terminate at a reverse proxy in front of Mango. Nginx or Apache in front of Mango (see our HTTP and HTTPS Settings guide) gives you a layer where header size limits are configurable (large_client_header_buffers in nginx, for example) before the request reaches Mango's embedded Jetty server at all. This is likely your most practical fix given the limit is coming from Jetty's defaults.
