Open the Settings app
Tap Display & Brightness
Tap Display Zoom
Select Larger Text
Tap Set
Confirm by tapping Use Zoomed
Open the keyboard to check the size
Turn on Display Zoom if needed
Go to Settings
Tap Display & Brightness
Tap View
Select Zoomed
Tap Set
Confirm by tapping Use Zoomed
Use one-handed keyboard mode only if you want a smaller keyboard
Rotate the iPhone to landscape mode for a larger keyboard
Use a third-party keyboard app with larger keys
Turn off Zoom if you want the default keyboard size back