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.)