The paper size is the dimensions of your output file. Paper size should always match your page/image size for correct PostScript file generation. Mismatching image and paper sizes is one of the most common problems in PostScript generation. Selecting a particular paper size for output does not mean that your job is scaled to fit to the desired output size.
Paper size creates a bounding area, like a frame, centered on your image. If the page/image size is larger than the selected paper size, then the parts of the page that do not fit into the paper size are cropped. If the page/image size is smaller than the selected paper size, the page centers on the paper with a border of white space surrounding the job.
It is common to select a paper size that fits the width of an output device, but to generate a PostScript file with a page/image size that was created at only 8.5" x 11". The result is that your image prints on a small area and is surrounded by a lot of wasted paper. You may need to create a custom paper/image size in your desktop application to correctly match the paper size of your output device.