Resolution:
We have no inherent limit in the size of messages that one can send (i.e., we don't have a hardcoded limit that we check against).  So the resources available to the clients and server limit it.  Anyway, attempting to send for example 513MB size message is not a good idea.  There are places where the message is copied which means holding 1GB in memory, not to mention the processor cycles/time needed to do the actual copy. The customer should break the message up or see if it is compressible.