how to set content length while building xml object using xml2js.Builder in nodejs

I am trying to understand xml2js module of node but I couldn't find much documentation on how to set content length for newly build XML if anyone has done earlier, please suggest the solution example : If i am tring to build xml as below

    var fs = require('fs'),
    xml2js = require('xml2js');
    var obj = {name: "Super", Surname: "Man", age: 23};
    var builder = new xml2js.Builder();
    var xml = builder.buildObject(obj);

Here i need to set content lenght how can i achieve this .

via Naveen Kumar