[{"isim":"This is topic - 2","resim":"anywhere2.png","ozellik":{"detay":["qv","asv","asd","vx"],"bilgi":["xc"]}},{"isim":"This is topic - 1","resim":"anywhere.png","ozellik":{"detay":["sa","as","de","ed"],"bilgi":["bv","cv","asd","dsa"]}}]

db.collection.aggregate([{$project:{_id:0,isim:1,boyut:{$strLenCP:{$ifNull:[{$arrayElemAt:["$ozellik.bilgi",-2]},""]}}}}])