Database
[{"_id":ObjectId("5a934e000102030405000000"),"associatedUsers":[ObjectId("5a934e000102030405000000"),ObjectId("5a934e000102030405000000")]},{"_id":ObjectId("5a934e000102030405000001"),"associatedUsers":[ObjectId("5a934e000102030405000000")]},{"_id":ObjectId("5a934e000102030405000002"),"associatedUsers":[ObjectId("5a934e000102030405000000"),ObjectId("5a934e000102030405000000"),ObjectId("5a934e000102030405000000")]}]
Query
db.collection.aggregate([{$addFields:{size:{$size:"$associatedUsers"}}},{$sort:{size:1}},{$limit:1}])