Create a mapping function:

map = function() { emit('msize', this.size); }

Create a reduce function:

red = function(k, v) {
  return Array.sum(v);
}

Run the mapReduce:

res = db.messages.mapReduce(map, red, {out: "Result"})

Get the result from the temporary collection called res

res.find()

{
  "_id": "msize",
  "value": 17318623
}