Open the Settings app
Tap Display & Brightness
Tap Display Zoom
Select Larger Text
Tap Done
Restart the iPhone if needed
Open Settings
Tap Accessibility
Tap Display & Text Size
Turn on Larger Text
Drag the slider to the right
Open an app with the keyboard to check the size
Rotate the iPhone to landscape mode for a larger keyboard
Use a third-party keyboard app with larger keys
Enable one-handed keyboard if needed by long-pressing the globe or emoji key and selecting left or right keyboard
Use Zoom in Accessibility to magnify the screen and keyboard