get the monitor size

Well, the tape measure may be the only accurate way to get this information :)

I tried several different approaches but the only one which produced reasonably accurate results was the WmiMonitorBasicDisplayParams class (http://msdn.microsoft.com/en-us/library/aa394535(VS.85).aspx). This uses E_EDID and so is only supported on Vista or later versions of Windows.

The C# code is as follows:

using System;
using System.Management;


class Test
{
    static void Main()
    {
       var searcher = new ManagementObjectSearcher("\\root\\wmi","SELECT * FROM WmiMonitorBasicDisplayParams");
       
       foreach(ManagementObject mo in searcher.Get())
       {
           double width = (byte)mo["MaxHorizontalImageSize"] / 2.54; 
           double height = (byte)mo["MaxVerticalImageSize"] / 2.54; 
           double diagonal = Math.Sqrt(width * width + height * height);
           Console.WriteLine("Width {0:F2}, Height {1:F2} and Diagonal {2:F2} inches", width, height, diagonal);            
       }
     
       Console.ReadKey();
    }
}

This gave a diagonal width of 15.33 ins on my Dell Laptop which actually has a diagonal of 15.6 ins. However, the properties in  question are rounded to the nearer centimeter and so could therefore have an error of plus or minus 0.2 inches which is not too far away.