Most Pages templates are created for documents with standard paper sizes, but you can change a document’s paper size at any time. You can also change its orientation (portrait or landscape). Afterward, you may need to make adjustments so that the document looks the way you want.
Tap 
at the top of the screen, tap Document Options, then tap Document Setup.
Tap a paper size.
In a word-processing document, you may need to tap the Document tab at the top of the screen to see the paper size controls.
When you’re finished, tap Done to return to the document.
You can also tap Custom Size to set custom paper sizes. See the task below for more information.
Tap at the top of the screen, tap Document Options, then tap Document Setup.
Tap a page orientation.
In a word-processing document, you may need to tap the Document tab at the top of the screen to see the page orientation controls.
Note: You can use only one page orientation for your document. Pages doesn’t support a mix of pages in portrait and landscape in the same document.
When you’re finished, tap Done to return to the document.
Tap at the top of the screen, tap Document Options, then tap Document Setup.
Tap Custom Size.
In a word-processing document, you may need to tap the Document tab at the top of the screen to see the paper size controls.
Tap Width or Height, then change the value to the new size.
To save your changes, tap Done in the top-right corner.
When you’re finished, tap Done to return to the document.