This is what I think you have to do to get a custom page size at least in OS X.x you need to select as the postscript printer a printer that supports custom size, the way to do this is is to add a virtual printer to the printer list and select Create Adobe PDF as the printer as it supports custom sizes. You will be able to choose custom page sizes and out put to PDF or ps.
Or you can in the case of 9x12 document use Xerox Docucolor 2006 which has a tabloid extra size which is 12x18 paper size.