It is not possible to resize a Flow or Org Chart Model.
The only workaround is to ungroup these types of Models so that you can resize them as a PICT. Although it still looks like a Flow or Org Chart, you will not be able to add new positions or use other Flow or Org Chart features. Given this, it is important not to ungroup any Model unless you are completely satisfied with it or you have made a copy of it.