Each message comes in as a String. Since each character in a string is 1 byte, just use String.length of each incoming message to determine the byte size of the message. You will probably need to add a trace somewhere inside the SmartFoxClient.as file to catch the incoming message.
Thanks for the input. That's one good method but is there a way to do it with some tools? Or can I actually use the debug method? But I will try with the solution you suggested. Thanks!