[{"key":1,"sweatersize":"Small"},{"key":2,"sweatersize":"Medium"},{"key":3,"sweatersize":"Large"},{"key":4,"sweatersize":"Large"},{"key":5,"sweatersize":"Large"},{"key":6,"sweatersize":"Medium"},{"key":7,"sweatersize":"Large"}]

db.collection.aggregate([{"$group":{"_id":"$sweatersize","count":{"$count":{}}}}])