Apply FontSize for the contents inside the run using Open XML SDK 2.0

 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using DocumentFormat.OpenXml;
using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Wordprocessing;
using System.IO.Packaging;
 
namespace OpenXMLConsole
{
    class Program
    {
        static void Main(string[] args)
        {
            string path = @"E:\OpenXMLFontSize.docx";
           
            //// Creates the new instance of the WordprocessingDocument class from the specified file
            //// WordprocessingDocument.Open(String, Boolean) method
            //// Parameters:
            //// string path - path is a string which contains the path of the document
            //// bool isEditable
            using (WordprocessingDocument doc = WordprocessingDocument.Create(path, WordprocessingDocumentType.Document)) 
            {
                //// Defines the MainDocumentPart              
 
                MainDocumentPart mainDocumentPart = doc.AddMainDocumentPart();
                mainDocumentPart.Document = new Document();
                Body body = mainDocumentPart.Document.AppendChild(new Body());
                Paragraph para = body.AppendChild(new Paragraph());
                Run run =para.AppendChild(new Run());
                RunProperties runProperties = run.AppendChild(new RunProperties());
                FontSize fontSize = new FontSize();
                fontSize.Val = "40";
                runProperties.AppendChild(fontSize);
                run.AppendChild(new Text("Welcome!!!!!"));
                mainDocumentPart.Document.Save();              
            }
        }
    }
}