Your title says you want a PNG (and you render a PostScript program to PNG, not "convert a PostScript image", PostScript is a programming language not an image format) but your description says you are creating PDF files. So which is it, PNG or PDF ?
Using PDFFitPage scales the requested media size to fit an actual (already set up, fixed) media size, since you haven't set a fixed media size on the command line, no scaling will be performed.
So, what media size are you getting, and why is that not correct ? I would 'guess' that your PostScript program does not request a media size, if it did then pdfwrite would create a PDF with the same media size.
In the absence of a media request, the pdfwrite device uses the default. Depending on your system and configuration that will most likely be either A4 or US Letter. It then reproduces the PostScript drawing program by either rendering to a bitmap, or as a PDF page description, using that media size. If the original PostScript required a different media size, then bits will be clipped.
Since you have not supplied an example its not possible for me to do more than guess of course.
However, you should probably try setting -sPAPERSIZE to something like A3. Or set a specific media size using -dDEVICEWIDTHPOINTS and -dDEVICEHEIGHTPOINTS.
If you supplied an example I could probably be more specific.
It would also be a decent idea to mention what version of Ghostscript you are using too.
