How to get exact figure size for post script figures

I save figures as post script files with the option -dpsc2. These files are later converted to PDF, e.g. by LaTeX or Acrobat Distiller. How can I control the exact size, e.g. 21cm x 29.7cm, of the final PDF? I succeed to get exact size of the final PDf if saved with "saveas" directly. However, I don't succeed when creating a PDF from a PS-file.