Find largest document in MongoDB and its size
db.CollectionName.find().toArray().map(function(request) {
return { size:Object.bsonsize(request), _id:request._id}; } )
.sort(function(a, b) { return a.size-b.size; }).pop();
Result (size is in bytes) // { "_id" : "DocumentUniqueId", "size" : 9999 }