How can I enlarge the keyboard on my iPad Air2

Hi Dr Caliban

Unfortunately there is no way for you to enlarge the iPad's keyboard. You are only able to modify the text of your device by going to settings; general; accessibility; larger text upon which you can toggle Larger Accessibility Sizes ON and then adjust the slider to your preferred text size.

You can also try to toggle the Bold Text setting ON which will make the text slightly easier to see.

Hope this helps!