How to set custom page size with Ghostscript

Question

I have scanned some materials at 600 dpi, ~ 9.36x12.67 inches and prepared in PostScript (PS) format.

Now when I try to transform PS to PDF with Ghostscript (GS),

Answer 1

I am using GhostPDL 9.10. Though my paper size of 8.5x13 is listed at http://ghostscript.com/doc/7.07/Use.htm#Known_paper_sizes as "flsa" the "-sPAPERSIZE=flsa" switch had no effect, the paper size remained Letter. The correct units for the -g5100x7800 switch matched my -r600 (not 720); -g works.