How can I have a custom Pages page size?
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.