How do I change the text / font size in iOS 7?

In iOS 6 you could change the font size of specific apps such as Mail, Messages and Notes via the Accessibility menu in settings.

In iOS 7 you can change the font size system wide by going to Settings -> General -> Text Size. You will see a slider that offers seven different text sizes. The text above the slider will change size to give you a preview as you slide to different positions. Choose the one that suits you best, and you're done. This will work with all native apps and all third party apps that support Dynamic Type.