SWIFT 3 - UILabel 글자 크기 부분 변경(Changing font size inside label)

import UIKit
 
class ViewController: UIViewController {
 
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        self.setLabel()
    }
 
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    
    func setLabel(){
        let label = UILabel(frame: self.view.frame)
        label.textAlignment = NSTextAlignment.center//중간정렬
        
        let bigBoldFont = UIFont.boldSystemFont(ofSize: 28.0)
        
        let attrString = NSMutableAttributedString(string: "Dr.kim의 나를 위한 블로그")
        attrString.addAttribute(kCTFontAttributeName as String, value: bigBoldFont, range: NSMakeRange(0, 6)) //0번째 글자부터 6글자
        
        label.attributedText = attrString
        
        self.view.addSubview(label)
    }
}