In my case I wanted the font to be of the same size. I set textField width constraint to >= 10 and it just works fine on iOS 9, 10, 11.
For iOS 11 this even works while editing text.
To make this work while editing text on iOS 9 and 10, I handled one more method by registering for a textDidChange event.
//Swift 3 Sample
//In viewDidLoad
adjustableTextField.addTarget(self, action: #selector(textFieldDidChange(_:)), for: .editingChanged)
@objc func textFieldDidChange(_ textField: UITextField) {
adjustableTextField.resignFirstResponder()
adjustableTextField.becomeFirstResponder()
}