How to Get PDF Height and Width

Hi Guys,
Can you please let me know how I can get the Height and Width of a PDF document using C#.I google this and i find out I can do this using the iTextSharp but I do not know how to do that.
What I have till now is:

 private void btnOpen_Click(object sender, EventArgs e)
        {
            OpenFileDialog dialog = new OpenFileDialog();
           
            if (dialog.ShowDialog() == DialogResult.OK)
            {
                txtFileName.Text = dialog.FileName;
                txtFileAdd.Text = dialog.SafeFileName;


                PdfReader reader = new PdfReader(dialog.SafeFileName);
                PdfImportedPage page = writer.GetImportedPage(reader, i);
                // size information
               txtFileWidth .Text= page.PageSize.Width;
               txtFileHeight .Text = page.PageSize.Height;
            }
           
        }
as you can see I am getting error on writer, which make sense but I do not know how to declare it!

Thanks alot