The iPhone and iPad have lots of settings for accessibility that makes it easier for people with hearing and visual impairments to use them. If you or someone you know has visual impairments, features such as Voice Over and enlarging the system text size can completely change their experience and help alleviate frustration.
Here's how to enlarge text size throughout iOS on iPhone and iPad:
That's it. You'll now notice that the text in apps such as Messages, Mail, and other default apps abides by the text size you set. While all apps can't abide by this, the ones that support it will.