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.
I'm currently playing around with XP Tablet edition 2005 for a customer of mine and can't seem to find a way to resize the osk.
The strange thing is when you press you press ctrl alt del the osk that appears there can be resize but you don't seem to get the same option in the normal desktop environment?