How can I get the active screen dimensions?

Screen.FromControl, Screen.FromPoint and Screen.FromRectangle should help you with this. For example in WinForms it would be:

class MyForm : Form
{
  public Rectangle GetScreen()
  {
    return Screen.FromControl(this).Bounds;
  }
}

I don't know of an equivalent call for WPF. Therefore, you need to do something like this extension method.

static class ExtensionsForWPF
{
  public static System.Windows.Forms.Screen GetScreen(this Window window)
  {
    return System.Windows.Forms.Screen.FromHandle(new WindowInteropHelper(window).Handle);
  }
}

You can use this to get desktop workspace bounds of the primary screen:

System.Windows.SystemParameters.WorkArea

This is also useful for getting just the size of the primary screen:

System.Windows.SystemParameters.PrimaryScreenWidth System.Windows.SystemParameters.PrimaryScreenHeight

Add on to ffpf

Screen.FromControl(this).Bounds

Beware of the scale factor of your windows (100% / 125% / 150% / 200%). You can get the real screen size by using the following code:

SystemParameters.FullPrimaryScreenHeight
SystemParameters.FullPrimaryScreenWidth

WinForms

For multi-monitor setups you will also need account for the X and Y position:

Rectangle activeScreenDimensions = Screen.FromControl(this).Bounds;
this.Size = new Size(activeScreenDimensions.Width + activeScreenDimensions.X, activeScreenDimensions.Height + activeScreenDimensions.Y);

This debugging code should do the trick well:

You can explore the properties of the Screen Class

Put all displays in an array or list using Screen.AllScreens then capture the index of the current display and its properties.

C# (Converted from VB by Telerik - Please double check)

        {
    List<Screen> arrAvailableDisplays = new List<Screen>();
    List<string> arrDisplayNames = new List<string>();

    foreach (Screen Display in Screen.AllScreens)
    {
        arrAvailableDisplays.Add(Display);
        arrDisplayNames.Add(Display.DeviceName);
    }

    Screen scrCurrentDisplayInfo = Screen.FromControl(this);
    string strDeviceName = Screen.FromControl(this).DeviceName;
    int idxDevice = arrDisplayNames.IndexOf(strDeviceName);

    MessageBox.Show(this, "Number of Displays Found: " + arrAvailableDisplays.Count.ToString() + Constants.vbCrLf + "ID: " + idxDevice.ToString() + Constants.vbCrLf + "Device Name: " + scrCurrentDisplayInfo.DeviceName.ToString + Constants.vbCrLf + "Primary: " + scrCurrentDisplayInfo.Primary.ToString + Constants.vbCrLf + "Bounds: " + scrCurrentDisplayInfo.Bounds.ToString + Constants.vbCrLf + "Working Area: " + scrCurrentDisplayInfo.WorkingArea.ToString + Constants.vbCrLf + "Bits per Pixel: " + scrCurrentDisplayInfo.BitsPerPixel.ToString + Constants.vbCrLf + "Width: " + scrCurrentDisplayInfo.Bounds.Width.ToString + Constants.vbCrLf + "Height: " + scrCurrentDisplayInfo.Bounds.Height.ToString + Constants.vbCrLf + "Work Area Width: " + scrCurrentDisplayInfo.WorkingArea.Width.ToString + Constants.vbCrLf + "Work Area Height: " + scrCurrentDisplayInfo.WorkingArea.Height.ToString, "Current Info for Display '" + scrCurrentDisplayInfo.DeviceName.ToString + "' - ID: " + idxDevice.ToString(), MessageBoxButtons.OK, MessageBoxIcon.Information);
}

VB (Original code)

 Dim arrAvailableDisplays As New List(Of Screen)()
    Dim arrDisplayNames As New List(Of String)()

    For Each Display As Screen In Screen.AllScreens
        arrAvailableDisplays.Add(Display)
        arrDisplayNames.Add(Display.DeviceName)
    Next

    Dim scrCurrentDisplayInfo As Screen = Screen.FromControl(Me)
    Dim strDeviceName As String = Screen.FromControl(Me).DeviceName
    Dim idxDevice As Integer = arrDisplayNames.IndexOf(strDeviceName)

    MessageBox.Show(Me,
                    "Number of Displays Found: " + arrAvailableDisplays.Count.ToString & vbCrLf &
                    "ID: " & idxDevice.ToString + vbCrLf &
                    "Device Name: " & scrCurrentDisplayInfo.DeviceName.ToString + vbCrLf &
                    "Primary: " & scrCurrentDisplayInfo.Primary.ToString + vbCrLf &
                    "Bounds: " & scrCurrentDisplayInfo.Bounds.ToString + vbCrLf &
                    "Working Area: " & scrCurrentDisplayInfo.WorkingArea.ToString + vbCrLf &
                    "Bits per Pixel: " & scrCurrentDisplayInfo.BitsPerPixel.ToString + vbCrLf &
                    "Width: " & scrCurrentDisplayInfo.Bounds.Width.ToString + vbCrLf &
                    "Height: " & scrCurrentDisplayInfo.Bounds.Height.ToString + vbCrLf &
                    "Work Area Width: " & scrCurrentDisplayInfo.WorkingArea.Width.ToString + vbCrLf &
                    "Work Area Height: " & scrCurrentDisplayInfo.WorkingArea.Height.ToString,
                    "Current Info for Display '" & scrCurrentDisplayInfo.DeviceName.ToString & "' - ID: " & idxDevice.ToString, MessageBoxButtons.OK, MessageBoxIcon.Information)

I Know it's an old question, but others may get some value from this

int formWidth = form.Width;
int formHeight = form.Height;
int formTop = form.Top;
int formLeft = form.Left;

Screen screen = Screen.PrimaryScreen;
Rectangle rect = screen.Bounds;
int screenWidth = rect.Width;
int screenHeight = rect.Height;
int screenTop = rect.Top;
int screenLeft = rect.Left;