How to calculate total size of the attachment?

 I write a sending email program but i don't know how and where to calculate total size of the attachment so that it won't pass the maximum length of the attachment...
my code is like this:
protected void btnAttach_Click(object sender, EventArgs e)
    {

string attachedFile = File1.PostedFile.FileName;

        if (attachedFile != "")
        {
            FileList.Items.Add(attachedFile);
        }
   }
 try
            {
                // check to see if there are any attachments or not
        &nb ...