How can I have a custom Pages page size? I'm trying to select, in the Documents tab, an A2 paper size so that it represents better the information that I will be printing out. However, there is no A2 paper size. I am also aware that under the Printing dialog (Paper Handling), there are more document types, but still no A2 size. 

Therefore, is there a way to make Pages visualise a custom paper size inside a document, and if yes, what is it?

Specs for reference: MacBook Pro (Retina, Early 2015) running OS X El Capitan v10.11.4 and Pages v5.6.2.
Using custom paper sizes actually falls to the printer driver, via the Page Setup window.


In Pages, select "Page Setup" from the File menu.
Click the "Paper Size" menu, and way at the bottom, select "Manage Custom Sizes".
This will present a window where you can define your page dimensions, give it a name, and save it for future use in the Page Setup window. (see screenshot) 


Once you've defined your paper size, it will always appear in the Page Setup window in the Paper Size dropdown menu, and Pages will use that size once selected.