Json web service max content length?

I have been building a Asp.net WCF web service with json format. Now I wanted to really test how its working when sending lots of data. The Content-Length of my http post is 65595. Directly when trying to connect I got error "HTTP/1.1 400 Bad Request" back. It seems like it's not even trying. I have been building a Asp.net WCF web service