Как рассчитать высоту (ширину) между 2 пикселями на экране?

Для этого необходимо глобально перехватывать низкоуровневые события мыши, см. LowLevelMouseProc, SetWindowsHookEx и другие функции, которые вы найдете ниже в примере кода.

Чтобы протестировать пример, создайте новый проект Windows Forms и добавьте три метки к основной форме, например:

И замените код в Form1.cs следующим:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Diagnostics;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            _proc = HookCallback;

            _hookID = SetHook(_proc);

        }

        // This method is called for each global mouse event
        private IntPtr HookCallback(
            int nCode, IntPtr wParam, IntPtr lParam)
        {
            if (nCode >= 0 &&
                MouseMessages.WM_LBUTTONDOWN == (MouseMessages)wParam)
            {
                MSLLHOOKSTRUCT hookStruct = (MSLLHOOKSTRUCT)Marshal.PtrToStructure(lParam, typeof(MSLLHOOKSTRUCT));
                if (isFirstClick)
                {
                    firstClickMouseStruct = hookStruct;
                    label1.Text = "First click: " + hookStruct.pt.x + ", " + hookStruct.pt.y;
                }
                else
                {
                    secondClickMouseStruct = hookStruct;
                    label2.Text = "Second click: " + secondClickMouseStruct.pt.x + ", " + secondClickMouseStruct.pt.y;
                    label3.Text = "Height: " + Math.Abs(secondClickMouseStruct.pt.y - firstClickMouseStruct.pt.y)
                        + ", Width: " + Math.Abs(secondClickMouseStruct.pt.x - firstClickMouseStruct.pt.x);
                }
                isFirstClick = !isFirstClick;
            }
            return CallNextHookEx(_hookID, nCode, wParam, lParam);
        }

        private MSLLHOOKSTRUCT firstClickMouseStruct;
        private MSLLHOOKSTRUCT secondClickMouseStruct;
        private bool isFirstClick = true;

        private LowLevelMouseProc _proc = null;
        private IntPtr _hookID = IntPtr.Zero;

        private IntPtr SetHook(LowLevelMouseProc proc)
        {
            using (Process curProcess = Process.GetCurrentProcess())
            using (ProcessModule curModule = curProcess.MainModule)
            {
                return SetWindowsHookEx(WH_MOUSE_LL, proc,
                    GetModuleHandle(curModule.ModuleName), 0);
            }
        }

        private delegate IntPtr LowLevelMouseProc(int nCode, IntPtr wParam, IntPtr lParam);

        private const int WH_MOUSE_LL = 14;

        private enum MouseMessages
        {
            WM_LBUTTONDOWN = 0x0201,
            WM_LBUTTONUP = 0x0202,
            WM_MOUSEMOVE = 0x0200,
            WM_MOUSEWHEEL = 0x020A,
            WM_RBUTTONDOWN = 0x0204,
            WM_RBUTTONUP = 0x0205
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct POINT
        {
            public int x;
            public int y;
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct MSLLHOOKSTRUCT
        {
            public POINT pt;
            public uint mouseData;
            public uint flags;
            public uint time;
            public IntPtr dwExtraInfo;
        }

        [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        private static extern IntPtr SetWindowsHookEx(int idHook,
            LowLevelMouseProc lpfn, IntPtr hMod, uint dwThreadId);

        [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        [return: MarshalAs(UnmanagedType.Bool)]
        private static extern bool UnhookWindowsHookEx(IntPtr hhk);

        [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        private static extern IntPtr CallNextHookEx(IntPtr hhk, int nCode,
            IntPtr wParam, IntPtr lParam);

        [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        private static extern IntPtr GetModuleHandle(string lpModuleName);

        private void Form1_FormClosed(object sender, FormClosedEventArgs e)
        {
            UnhookWindowsHookEx(_hookID);
        }
    }
}

Теперь вы можете щелкнуть левой кнопкой мыши внутри или за пределами формы (на рабочем столе или в других окнах), и приложение перехватит клики, проверит положение мыши и рассчитает высоту и ширину по мере необходимости: