[{_id:"1",attributes:[{a:1},{b:2}],collection_name:"A"},{_id:"2",attributes:[{b:2}],collection_name:"B"},{_id:"3",attributes:[{a:1},{b:2},{b:2},{b:2}],collection_name:"A"},]

db.collection.aggregate([{$addFields:{maxLengthOfArray:{$size:"$attributes"}}},{$sort:{maxLengthOfArray:-1}},{$limit:1},{$project:{_id:1,attributes:1,maxLengthOfArray:1}}])