You could add it yourself Daniel; in the Paper Size, srcoll to the bottom and select Manage Custom Sizes and create an A3 which is 420 mm X 297mm, and save it as A3
I did that, but it remained grayed out in the Paper Size menu of the Print dialog box and I'm unable to select it.
Ah-hah - when I select a different printer, I
can select A3. So it's something to do with the Okidata C9500 printer driver... (Unfortunately, the different printer is not big enough to accept A3 paper.)