pNova.SetOptionString2(NOVAPDF_PAGE_FORM, 'what to insert here?', PROFILE_NAME, PROFILE_IS_PUBLIC);
In the documentation is to be read that anything like "A4", "Letter", a.s.f. can be inserted but I'm not able to find a gathering of all possible strings. Inserting 'funny bunny' won't give me a bunny shaped paper format, will it?
The "page form" setting cannot be used to change the page setting to a specific form, it can only be used to read the form set on printer. Here is an article on how to change paper size programatically: