PostScript Printer Driver in Mac OS 10.6.8

Ok, I'm going nuts here ! I have a Mac OS 10.6.8 on my Macbook Pro, and I'm using InDesign CS5 to create a document in PDF. I can export to PDF but I've been told that option makes the file larger comparing to when you print to PDF, and I need to make the final PDF smaller than 8 MB. I tried everything I could to make all the images and everything smaller but the PDF is still big. Now the only thing that's left and I haven't tried is printing to PDF. I have no idea why on earth the custom paper size option is grayed out in the PostScript driver I have on my mac and I've googled and read every thread I found remotely related and I can't find a straight answer. Is it really impossible to print to PDF from MacBook Pro using a PostScript driver that has custom size paper option available ? Any help is really appreciated. Thank you !