While using any app on iPhone, you can increase or decrease the text size. You can either change the text size within the app you're using or change the text size across all your other apps simultaneously.
Change text size
Open Control Center by swiping up and tap the control aA - If you do not see aA, add it to the Control Center by going to Settings > Control Center, then select Text Size
Drag the slider up or down to increase or decrease the text size
To change the text size for all apps, tap All Apps on the bottom right of the screen