Using maven jetty plugin version: 9.1.3.v20140225
You are going to need a custom value for your "request buffer size". By default it is 4096 bytes.
You can use the method setRequestBufferSize of the HttpClient class (see javadoc: org.eclipse.jetty.client.HttpClient)
HttpClient httpClient = new HttpClient();
httpClient.setRequestBufferSize(/*your custom value*/);
httpClient.start();
ContentResponse response = httpClient.POST("http://0.0.0.0:" + host.getPort() + "/store").param("p", jsonValue).send();
Your custom value will depend on how many bytes you are sending plus a few more, which are always sent by the HTTP client implementation. For example, in my case and using the above code I am sending the following:
POST /?p=YOUR_JSON_VALUE HTTP/1.1
Accept-Encoding: gzip
Host: 0.0.0.0:8080/store
User-Agent: Jetty/9.1.3.v20140225