Within the iOS Accessibility Options you can increase or decrease the size of the system wide font. This will mean that wherever it's supported in iOS (most of the Apple stock apps, menu's, Notification Center and third party apps where they have used the font option) the text will be as you want it I.e. Smaller or Bigger than the standard size that ships with iOS.
I cranked the size down on both my iPad Pro and iPhone 6S and have been enjoying the smaller font size, here is how you do it.
- Open Settings
- Tap General
- Tap Accessibility
- Tap Larger Text
- You can now drag the slider to increase or decrease the font size