Currently there is no way of changing page size and margins of the document except for diving into OOXML. It is a very common use case for an add-in to be able to have read/write access to page size ...
More research has revealed that this is not even possible with OOXML. So there is literally no way of changing page size, orientation or margins using Office.js. This is very basic functionality and should have been supported from the very first version. Kindly look into it and suggest a workaround or add this feature to the API.