[{"key":1,biddings:[1,2]},{"key":2,biddings:[5]},{"key":3,biddings:[1,2]},{"key":4,biddings:[7,8,9,10]},{"key":5,biddings:[2,5]},{"key":6,biddings:[9,8,7,6,5,4,3,2,1]}]

db.collection.aggregate([{"$addFields":{"arrayCount":{$size:"$biddings"}}},{$sort:{arrayCount:-1}}])