Database
[{"_id":ObjectId("529159af5b508dd71500000a"),"c":"somecontent","l":[{"d":"2013-11-24T01:43:11.367Z","u":"User1"},{"d":"2013-11-24T01:43:51.206Z","u":"User2"}]}]
Query
db.collection.aggregate([{$addFields:{total:{$size:"$l"}}},{$sort:{total:-1}},{$limit:25}])