You can make your own

Copy following two files to a folder.

To Use:

GetWindowRect <Title Of Window>

EG

GetWindowRect Untitled - Notepad

Change /target:exe to /target:winexe to make it a non console program

REM GetWindowRect.bat
REM This file compiles GetWindowRect.vb to GetWindowRect.exe
REM GetWindowRect.exe reports on Windows position
REM To use 
REM GetWindowRect 
"C:\Windows\Microsoft.NET\Framework\v4.0.30319\vbc.exe" /target:exe /out:"%~dp0\GetWindowRect.exe" "%~dp0\GetWindowRect.vb"
pause

;GetWindowRect.vb
imports System.Runtime.InteropServices 
Public Module GetWindowRect  

     <StructLayout(LayoutKind.Sequential)> _
    Private Structure RECTL     
        Public Left As UInt32
        Public Top As UInt32
        Public Right As UInt32
        Public Bottom As UInt32
    End Structure

    Private Declare Function GetWindowRect Lib "User32" (ByVal hWnd as IntPtr, ByRef Rect as RectL) as Integer
    Public Declare UNICODE Function FindWindowW Lib "user32" (ByVal lpClassName As String, ByVal lpWindowName As String) As IntPtr

    Sub Main
        On Error Resume Next
        Dim hWindows as IntPtr
        Dim Ret as Integer
        hwindows = FindWindowW(vbNullString, Command())
        If hwindows = 0 then
            Msgbox(Command() & " cannot be found.")
        Else
            Dim x as RectL
            Ret = GetWindowRect(hWindows, x)
            If Ret = 0 Then 
                MsgBox("GetWindowRect Error " & Err.LastDllError)
            Else
            'Delete the MsgBox line if using as console program
            Msgbox(x.left & " " & x.top & " " & x.right & " " & x.bottom)
            Console.Writeline(x.left & " " & x.top & " " & x.right & " " & x.bottom)
            End If
        End If
    End Sub

End Module 

Also available here - https://winsourcecode.blogspot.com/2020/01/getwindowrectexe-reports-on-windows.html

This has everything you need to know: Get viewport/window size

but in short:

var win = window,
    doc = document,
    docElem = doc.documentElement,
    body = doc.getElementsByTagName('body')[0],
    x = win.innerWidth || docElem.clientWidth || body.clientWidth,
    y = win.innerHeight|| docElem.clientHeight|| body.clientHeight;
alert(x + ' × ' + y);

Fiddle

Please stop editing this answer. It's been edited 22 times now by different people to match their code format preference. It's also been pointed out that this isn't required if you only want to target modern browsers - if so you only need the following:

const width  = window.innerWidth || document.documentElement.clientWidth || 
document.body.clientWidth;
const height = window.innerHeight|| document.documentElement.clientHeight|| 
document.body.clientHeight;

console.log(width, height);