custom output extent and spacing in vtkImagePlaneWidget

hi, I am using vtkImagePlaneWidget because it has the window leveling, cursor and text display all bundled in. However, I have a question, is there a way to make the vtkImagePlaneWidget output a specific size and resolution ?
I have access to a GetReslice() method on the vtkImagePlaneWidget but doing something like the below did not wok:

  double[] sp = new double[] { 0.4, 0.4, 0.4 };
  int dimX = 400;
  int dimY = 500;

planeWidget.GetReslice().SetOutputSpacing(sp[0], sp[1], sp[2]);
planeWidget.GetReslice().SetOutputExtent(0, dimX-1, 0, dimY-1, 0, 0);
planeWidget.GetReslice().SetOutputOrigin( - 0.5*(dimX - 1) * sp[0], - 0.5 * (dimY - 1) * sp[1], 0.0);

Any idea on how I can make the output extent, spacing and origin of the imagePlaneWidget to what I want such as the above rather than the extent being fixed by the input data extent ?