Changing font size via c#

Thanks zoltan for the reply. My requirement is that, the company's name in the top should be in 12pt while rest of the contents like Username and Phone No should be in 21 pt.
Thank you.

Thanks David. Below is my code:
Microsoft.Office.Interop.Word.Table tbl1;
Microsoft.Office.Interop.Word.Range wordRange = doc.Content;
tbl1 = doc.Content.Tables.Add(wordRange, 3, 2, ref objMiss, ref objMiss);//3 columns, 2 rows
tbl1.Borders.Enable = 1;

int cc = 1;
for (int r = 1; r <= TotalNumber; r++)//TotalNumber is some number
{
for (int c = 1; c <= 2; c++)
{

if (cc < UserNameOne.Length) //UserNameOne is a String Collection
{
tbl1.Cell(r, c).Range.Text = "Welcome to XYZ Company\n" + "User Name:" + UserNameOne[cc] + "\n" ;//I want the size of "Welcome to XYZ Company" to be 12 pt
tbl1.Cell(r, c).Range.Font.Bold = 0;
tbl1.Cell(r, c).Range.Font.Size = 21;
cc = cc + 1;
}
}