> cscript //Nologo du.js "C:\test_dir"

du.js

function getDirectorySize(path) {
    var fd = new ActiveXObject("Scripting.FileSystemObject");
    var d;

    try {
        d = fd.GetFolder(path);
        return d.Size;
    } catch(e) {
        return -1;
    }
}

var arg = WScript.arguments;
if (arg.length!=1) {
    WScript.echo(-1);
} else {
    WScript.echo(getDirectorySize(arg(0)));
}