// 方法一：

From2 frm2 = new From2();
if (Screen.AllScreens.Count() != 1)
{
  frm2.Left = Screen.AllScreens[0].Bounds.Width;
  frm2.Top = 0;
  frm2.Size = new System.Drawing.Size(Screen.AllScreens[1].Bounds.Width, Screen.AllScreens[1].Bounds.Height);
}
// 方法二：
   this.Left = ((Screen.AllScreens[1].Bounds.Width - this.Width) / 2);
   this.Top = ((Screen.AllScreens[1].Bounds.Height - this.Height) / 2);
   this.Size = new System.Drawing.Size(Screen.AllScreens[1].Bounds.Width, Screen.AllScreens[1].Bounds.Height);