In case you are still searching for a better solution, I think this is what you want:

A Boolean value indicating whether the font size should be reduced in order to fit the title string into the label’s bounding rectangle (this property is effective only when the numberOfLines property is set to 1).

When setting this property, minimumScaleFactor MUST be set too (a good default is 0.5).

Swift

var adjustsFontSizeToFitWidth: Bool { get set }

Objective-C

@property(nonatomic) BOOL adjustsFontSizeToFitWidth;

A Boolean value indicating whether spacing between letters should be adjusted to fit the string within the label’s bounds rectangle.

Swift

var allowsDefaultTighteningForTruncation: Bool { get set }

Objective-C

@property(nonatomic) BOOL allowsDefaultTighteningForTruncation;

Source.

This is how I get UILabel Autoshrink work (especially for height of label font fitting in 4s device from 6s Plus Storyboard) in iOS 9.2, Xcode 7.2 ...

• Number of lines is 0
• Line Breaks: Clip
• Autoshrink: Minimum Font Scale 0.25

minimumFontSize is deprecated in iOS 6.

So use minimumScaleFactor instead of minmimumFontSize.

lbl.adjustsFontSizeToFitWidth = YESlbl.minimumScaleFactor = 0.5

Swift 5

lbl.adjustsFontSizeToFitWidth = truelbl.minimumScaleFactor = 0.5