I can only assume you are using an iPhone. Anyway, try the following:
1. Reboot your phone.
2. Go to Settings - General - Accessibility and then turn OFF "Zoom".
3. Go to Settings - General - Accessibility - Larger Text and then either turn it OFF or slide the lever to an appropriate position.
4. Go To Settings - Display & Brightness - Text Size and then slide the lever to the appropriate position.