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);
  }
}