I couldn't find anything about this in Google other than what you have found with the log-in screen, but if no-one else here has any ideas, have a look at "On-Screen Keyboard Magic" from;

It seems this allows the user to resize Microsoft's on-screen keyboard as well as having other features.

I am not sure if this is compatible with the XP Tablet, and if you want to be sure you may have to contact the developers.

It s not free, but is available for a free trial.

I hope that helps.