使用SystemInformation类

//当前的屏幕除任务栏外的工作域大小
string currentScreenSize_OutTaskBar=SystemInformation.WorkingArea.Width.ToString() + "," +SystemInformation.WorkingArea.Height.ToString();
MessageBox.Show("当前的屏幕除任务栏外的工作域大小为:"+currentScreenSize_OutTaskBar);

//当前的屏幕包括任务栏的工作域大小
string currentScreenSize=System.Windows.Forms.Screen.PrimaryScreen.Bounds.Width.ToString() + "," + System.Windows.Forms.Screen.PrimaryScreen.Bounds.Height.ToString();
MessageBox.Show("当前的屏幕包括任务栏的工作域大小为:"+currentScreenSize);

//任务栏大小
Size OutTaskBarSize = new Size(SystemInformation.WorkingArea.Width, SystemInformation.WorkingArea.Height);

Size ScreenSize = new Size(System.Windows.Forms.Screen.PrimaryScreen.Bounds.Width, System.Windows.Forms.Screen.PrimaryScreen.Bounds.Height);

Size TaskBarSize;

TaskBarSize = new Size(
                (ScreenSize.Width - (ScreenSize.Width - OutTaskBarSize.Width)),
                (ScreenSize.Height - OutTaskBarSize.Height)
                );

MessageBox.Show("任务栏大小：" + TaskBarSize.Width + "," + TaskBarSize.Height);