WhatsApp uses the text size you select in your iPhone's settings. To change the text size of messages within WhatsApp:

1. Go to iPhone Settings.
2. Tap Display & Brightness > Text Size.
3. Move the slider to adjust the text size.

Note:

• If you turn on Larger Accessibility Sizes in iPhone Settings > General > Accessibility > Larger Text, you'll no longer be able to see profile photos and group icons in WhatsApp. This behavior matches with iPhone Messages to provide an easier-to-read display.
• WhatsApp doesn't support custom fonts.