To make the text size bigger or smaller on an iOS device: 

1. Open your iPhone’s Settings app 
2. Select Display & Brightness
3. Select Text Size
4. Use the slider to select the font size you want

You can also turn on Larger Accessibility Text Sizes on iOS:

1. Open your iPhone’s Settings app 
2. Select Accessibility  
3. Select Display & Text Size 
4. Select Larger Text 
5. Select the Larger Accessibility Sizes toggle switch 
6. Use the slider to select the font size you want

To make the text size bigger or smaller on an Android device: 

1. Open your device’s Settings app 
2. Select Display (or Display & touch or Display & brightness)  
3. Select Font size and style (or Display size and text or Font) 
4. Use the font size slider to adjust the settings