[{"tags":[1,2]},{"tags":[1]},{"tags":[1,2,3]},{"tags":[]},{"tags":"aaa"},{"tags":false},{"tags":0}]

db.collection.aggregate({"$match":{"$expr":{"$isArray":"$tags"}}},{"$addFields":{"aux_size":{"$size":"$tags"}}},{"$sort":{"aux_size":1}},{"$project":{"aux_size":0}})