{$strLenBytes: <expression>}
{$strLenCP: <expression>}  // CP stands for UTF-8 'code points'.

{_id: 0, string: "cliché"}

db.coll.aggregate([{
    $project: {
        byteLength: {$strLenBytes: "$string"},
        cpLength: {$strLenCP: "$string"}
    }
}])

{_id: 0, byteLength: 7, cpLength: 6}

db.kol.insert({"text": "abcde"})
db.kol.insert({"text": "ab"})

db.kol.aggregate({ $project: { "text_length": {$length: "$text"  }}})

{ "_id" : ObjectId("53d0c9bdc2644cdc0ab835f5"), "text_length" : 5 }
{ "_id" : ObjectId("53d0c9c1c2644cdc0ab835f6"), "text_length" : 2 }