Why has my iPad keyboard gone small?

Have you enabled the Scribble feature? This feature uses a different mini-keyboard that does not respond to the two-finger zoom or pinch gestures.

Settings > Apple Pencil > Scribble - set to OFF

Otherwise, to restore the Floating Keyboard to the normal on-screen keyboard presentation, use the two-finger “zoom” gesture (i.e., the reverse of the two-finger “pinch”).

If the Split Keyboard is restored instead of the full on-screen keyboard, you can either use two fingers (one on each of the two half-keyboards) and swipe the two halves together - or touch-and-hold the “keyboard” key, then slide your finger to the Merge & Dock option and release.

Use the floating keyboard on your iPad – Apple Support

Split and move the keyboard on your iPad – Apple Support

Force shutdown the device then restart normally and test. If the keyboard is still small then look in your Accessibility settings, something was likely changed by you in-error.