If that's still not big enough, install the Developer Tools. Then go to:
/Developer/Applications/Performance Tools/Quartz Debug.app
(menu bar) -> Tools -> Show User Interface Resolution
Slide the bar up a bit from 1.0, then close & reopen the Keyboard Viewer. Remember to set the slider back to 1.0 when you are done, or a number of other applications will look odd the next time they are launched.