Well that is an interesting question. So the operation-data is in memory, as part of the event document, so no writes to disk, which would cost time, and then as it is submitted to the shim, it is stripped and restored when the event returns. But as far as I know that too is cached in memory.
So I do not know if there is any specific limit, but I imagine anything likely should be ok. That is, if you come up with a case with 20Megs of Op-data I cannot imagine that would be wise. But a few K I do not think would be an issue.
An excellent question, and even better is, how would one find out the answer definitively? Hmm..