How do I define the page size when using Publish

Please excuse the newbie question, but I am learning Matlab and want to use Publish to create an assignment report.

I'm using R2013a on a Mac, so don't have the option to publish to Word. Instead I'm publishing to PDF, but it appears to default to US Letter paper size when I need it to use A4.

Where do I specify the page size? I've looked in preferences and in Publish options but to no avail.

Thanks in advance.