At the keyboard, long press the &123 button ad it'll take you to keyboard settings and press the cog wheel. Then go to more keyboard settings and there you can adjust the size.
My keyboard has been offset to the right for a couple of weeks now. I found the S-M-L in the Time & language>Keyboard>More keyboard settings page but the &123 long press brings you to the keyboard offset setting as well (lower button on the popup). Wow! My keyboard is centered again!