Set a custom font size for any app on iPhone and iPad
Most iPhone and iPad users know you can adjust font size globally by going to Settings > Display & Brightness > Text Size. But you can also change it for a single app without touching anything else.
Here's how:
1. Open the app you want to adjust.
2. Open Control Center -- swipe down from the top-right corner.
3. Tap the Text Size icon.
4. Select the App Only option.
5. Move the slider to adjust the size.
More white bars means larger text. There's a preview at the top of the screen so you can see the change in real time.