self.mybutton.titleLabel.minimumScaleFactor = 0.5f;
self.mybutton.titleLabel.numberOfLines = 0; <-- Or to desired number of lines
self.mybutton.titleLabel.adjustsFontSizeToFitWidth = YES;
… did the trick, after layoutIfNeeded in viewDidLoad
As it turns out, all those must be set to actually adjust the font-size, not just making it fit into the frame.
Update for Swift 3:
mybutton.titleLabel?.minimumScaleFactor = 0.5
mybutton.titleLabel?.numberOfLines = 0 <-- Or to desired number of lines
mybutton.titleLabel?.adjustsFontSizeToFitWidth = true