What is the max length of the content when resending a message

We have a service that sends XML message. During unit test, I tried to resend the message with following scenario:

(1) If I leave the content blank, the original message will be resent in the ContentStream;

(2) If I post a new XML message, the new XML will be sent in the Content, the original message will be resent in the ContentStream;

(3) If I post a new XML message with very big length, it will be converted to 5@%CSP.CharacterStream, and following kind of error will be thrown:

ERROR #6901: XSLT XML Transformer Error: SAXParseException: invalid document structure (Occurred in an unknown entity, at line 1, column 23.)

My questions are why resending the original message even I post a new message in the content? Is there any setting to avoid it? In addition, what is the maximum length of the content?

Thanks.