Hmm.. at last found the answer.. I will post C# Code for Setting Page Size.
using Word = DocumentFormat.OpenXml.Wordprocessing;
WordprocessingDocument WordDoc = WordprocessingDocument.Create(SavePath, WordprocessingDocumentType.Document);
MainDocumentPart mainDocument = WordDoc.AddMainDocumentPart();
mainDocument.Document = new Word.Document();
Word.Body body = new Word.Body();
Word.SectionProperties SecPro = new Word.SectionProperties();
Word.PageSize PSize = new Word.PageSize();
PSize.Width = 15000;
PSize.Height = 11000;
SecPro.Append(PSize);
body.Append(SecPro);
body.Append(WordTable);
mainDocument.Document.Append(body);
mainDocument.Document.Save();
WordDoc.Close();
and Rubens thanks for reply. But I knew about PageSize. I was searching for how and where to insert it. I found it using method you specified in another question(By opening Word document using Archive manager).
Only problem here is conversion between pixel and Twips.(My measurements are in pixel) Hmm I have to do something else for that..
(I found a multiplication factor of 10 using try and error method which should be used with measurement in pixel for converting into Twips. Don't know this is correct. But works fine for me.)