Which program is this in? Often you need to set the machine to be using the printer before you go to the print dialogue in order to make the larger paper sizes available but how depends on the program.
I'm pretty sure ora had in mind that you should select the "format for: your printer" in Page Setup before trying to go to the print dialog. Did you do that?
Yep that was what I meant gsahli - I have had a simialr issue with other large scale porinters before.
You canalso try PrintTherapy which a fellow macosxer suggested to me to fix a print issue. It costs but there is a 7 say trial you can use for free, and it seems to fix a lot of OS X print issues.