as previously stated there is no specific message size limit. The largest message size the system will process is mainly dependent on : system resources, tuning and throughput.
As michal suggested the recommended one might is 5MB. But in one of the presentation from Geroge Yu he told XI has processed around 35MB message for one of the client. It all depends upon the hardware you use and how you tune it.
You will not have the exact document what you are looking for. I think we dont have that. But to some extent if you read this you can have some idea on this:
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