If you change the font size on your iPhone, it affects every default apps on your phone as well as the 3rd party apps that support Dynamic type. To change font size on iPhone 6 and other iPhone, iPod touch and iPad devices, do as follows:
1. Open the Settings application on your iPhone
2. Go to the Display & Brightness settings
3. There you get to change (decrease or increase) the text size, as well as making the text Bold.
4. Just tap the Text Size option and use the slider to adjust the text font size on iPhone.
Tip: If you need a bigger text size on iPhone, then follow these steps:
1. Open the Settings app and go to the General settings
2. Find the Accessibility settings (in the middle of the screen) and open it
3. Tap the Larger Text option
4. Now make sure the Larger Accessibility Sizes is toggled ON, and then use the slider and drag it to the right for even bigger font size on iPhone.