Hi Ivy,
The page size is controlled per section (as the control supports different page sizes per document) and is accessible from the page size property - for example:
section.PageSize = new NPageSize(ENPaperKind.A4);
You can also take a look at the following example:
Text Editor \ Sections \ Pages.
Hope this helps - let us know if have any questions...
Best Regards,
Nevron Support Team