Get Display Size

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var hDC = GetDC(IntPtr.Zero);
            var vertSize = GetDeviceCaps(hDC, DeviceCap.VERTSIZE);
            var horzSize = GetDeviceCaps(hDC, DeviceCap.HORZSIZE);
            Console.WriteLine("{0}x{1}mm", horzSize, vertSize);
            Console.Read();
        }

        [DllImport("User32.dll")]
        static extern IntPtr GetDC(IntPtr hwnd);

        [DllImport("gdi32.dll")]
        static extern int GetDeviceCaps(IntPtr hdc, DeviceCap nIndex);
        public enum DeviceCap
        {
            /// <summary>
            /// Device driver version
            /// </summary>
            DRIVERVERSION = 0,
            /// <summary>
            /// Device classification
            /// </summary>
            TECHNOLOGY = 2,
            /// <summary>
            /// Horizontal size in millimeters
            /// </summary>
            HORZSIZE = 4,
            /// <summary>
            /// Vertical size in millimeters
            /// </summary>
            VERTSIZE = 6,
            /// <summary>
            /// Horizontal width in pixels
            /// </summary>
            HORZRES = 8,
            /// <summary>
            /// Vertical height in pixels
            /// </summary>
            VERTRES = 10,
            /// <summary>
            /// Number of bits per pixel
            /// </summary>
            BITSPIXEL = 12,
            /// <summary>
            /// Number of planes
            /// </summary>
            PLANES = 14,
            /// <summary>
            /// Number of brushes the device has
            /// </summary>
            NUMBRUSHES = 16,
            /// <summary>
            /// Number of pens the device has
            /// </summary>
            NUMPENS = 18,
            /// <summary>
            /// Number of markers the device has
            /// </summary>
            NUMMARKERS = 20,
            /// <summary>
            /// Number of fonts the device has
            /// </summary>
            NUMFONTS = 22,
            /// <summary>
            /// Number of colors the device supports
            /// </summary>
            NUMCOLORS = 24,
            /// <summary>
            /// Size required for device descriptor
            /// </summary>
            PDEVICESIZE = 26,
            /// <summary>
            /// Curve capabilities
            /// </summary>
            CURVECAPS = 28,
            /// <summary>
            /// Line capabilities
            /// </summary>
            LINECAPS = 30,
            /// <summary>
            /// Polygonal capabilities
            /// </summary>
            POLYGONALCAPS = 32,
            /// <summary>
            /// Text capabilities
            /// </summary>
            TEXTCAPS = 34,
            /// <summary>
            /// Clipping capabilities
            /// </summary>
            CLIPCAPS = 36,
            /// <summary>
            /// Bitblt capabilities
            /// </summary>
            RASTERCAPS = 38,
            /// <summary>
            /// Length of the X leg
            /// </summary>
            ASPECTX = 40,
            /// <summary>
            /// Length of the Y leg
            /// </summary>
            ASPECTY = 42,
            /// <summary>
            /// Length of the hypotenuse
            /// </summary>
            ASPECTXY = 44,
            /// <summary>
            /// Shading and Blending caps
            /// </summary>
            SHADEBLENDCAPS = 45,

            /// <summary>
            /// Logical pixels inch in X
            /// </summary>
            LOGPIXELSX = 88,
            /// <summary>
            /// Logical pixels inch in Y
            /// </summary>
            LOGPIXELSY = 90,

            /// <summary>
            /// Number of entries in physical palette
            /// </summary>
            SIZEPALETTE = 104,
            /// <summary>
            /// Number of reserved entries in palette
            /// </summary>
            NUMRESERVED = 106,
            /// <summary>
            /// Actual color resolution
            /// </summary>
            COLORRES = 108,

            // Printing related DeviceCaps. These replace the appropriate Escapes
            /// <summary>
            /// Physical Width in device units
            /// </summary>
            PHYSICALWIDTH = 110,
            /// <summary>
            /// Physical Height in device units
            /// </summary>
            PHYSICALHEIGHT = 111,
            /// <summary>
            /// Physical Printable Area x margin
            /// </summary>
            PHYSICALOFFSETX = 112,
            /// <summary>
            /// Physical Printable Area y margin
            /// </summary>
            PHYSICALOFFSETY = 113,
            /// <summary>
            /// Scaling factor x
            /// </summary>
            SCALINGFACTORX = 114,
            /// <summary>
            /// Scaling factor y
            /// </summary>
            SCALINGFACTORY = 115,

            /// <summary>
            /// Current vertical refresh rate of the display device (for displays only) in Hz
            /// </summary>
            VREFRESH = 116,
            /// <summary>
            /// Horizontal width of entire desktop in pixels
            /// </summary>
            DESKTOPVERTRES = 117,
            /// <summary>
            /// Vertical height of entire desktop in pixels
            /// </summary>
            DESKTOPHORZRES = 118,
            /// <summary>
            /// Preferred blt alignment
            /// </summary>
            BLTALIGNMENT = 119
        }
    }
}

THANKS FOR COMMENTS!!!.