Sometimes the tablet drawing area doesn't map to the Screen size correctly - most of the time what happens is that the drawing area is too small, so you can only draw in a rectangle in a middle of the screen. Sometimes it happens on new Nodekit installations, or after changing the tablet model. There's a workaround - go to the CameraFitToContent node that leads into the Display, and adapt the "Norm" parameter. But it should automatically fit the tablet drawing area to the screen, so that it covers it completely, keeping intact the ratio of the tablet - so if necessary some tablet areas will be outside the view.
Here's a thread of a user reporting this problem: http://groups.google.com/group/tagtool/browse_thread/thread/3829847613fafbb8/aefc90ded59401a1?lnk=gst&q=drawing+size#aefc90ded59401a1