Choose the Page tool. It's used for creating new page sizes in the document.
In the Control panel, from the page size preset popup menu, choose Custom. In the Custom Page Size dialog, create a new size like 6" x 6" and save it. Then it will be added to the page size menu.
If you then create a master page of that size, you can drag into the document page section of the Pages panel to create pages of that size.
It may depend on what kind of digital document you're trying to create: Interactive PDF, EPUB, other?
It'd be an Interactive PDF.
be aware of the following bug documented here:
Master Page Items Moving on Custom-Sized Page
JustBob May 16, 2014
Mike (user mikescopino) provided a very good discription of the bug here:
I could recreate the bug, that will hit you, if you are using the edit fields of the Page Tool to change the size of the page. And duplicate that page:
So I'd recommend, if you are using the Page Tool to resize the page, use the mouse to change the size of the page and not the value edit fields of the tool.
InDesign CC, CC-2014 and CC-2015.2.
In CS6, CS5.5 or CS5 I cannot see this bug.