[{"_id":1,"item":"ABC",price:NumberDecimal("80"),"sizes":["S","M","L"]},{"_id":2,"item":"EFG",price:NumberDecimal("120"),"sizes":[]},{"_id":3,"item":"IJK",price:NumberDecimal("160"),"sizes":["M"]},]

db.collection.aggregate([{$match:{$expr:{$ne:[{"$size":"$sizes"},0]}}}])