Different font size for different devices

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Adaptive Layout Tutorial in iOS 9: Getting Started

Select your text field in your storyboard. In the attributes inspector you will see a small '+' to the left of the font selector. This will allow you to choose size-class specific stuff for your text.

Size classes were added to iOS 8 when Apple introduced Universal Storyboards which let you design for both iPad and iPhone with the same Storyboard file. At the bottom of the Storyboard design window you'll see the (default) 'w Any h Any' button. This gives you access to combinations of the compact and regular size classes. These available combinations let you implement design, say for only the iPhone in landscape, or maybe the iPad in both orientations. You have full control.

For your font related question, you access the size-class control through the '+' sign I mentioned above.

This is a good explanation and tutorial: Wenderlich size classes

and in Apple's own words: Apple size classes