If a message with <6k bytes does through, but messages with >6k don't, I'd take a look at the client and host limits for things like:
MaxBufferSize
MaxBufferPoolSize
MaxReceivedMessageSize
You don't say whether or not you have control over the host server settings, but you can increase and decrease the limits on items like those mentioned earlier. You can set them to Integer.Max if necessary, a size that would allow file uploads > 1 GB.