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

db.collection.aggregate({"$addFields":{"aux_size":{"$cond":{"if":{"$isArray":["$tags"]},"then":{"$size":"$tags"},"else":0}}}},{"$sort":{"aux_size":1}},{"$project":{"aux_size":0}})