Description
On the map/reduce page, some size limitations are mentioned (e.g. emit can only output 1/2 of BSON size), but it doesn't explicitly state that the reduce function is also bound by the 1/2 BSON size = 8MB limit.
The error you get when trying to return a document larger than that limit is
"errmsg" : "exception: value too large to reduce",
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"code" : 13070,
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This should be stated in the list of requirements for the reduce function.