Get size of primary monitor by PowerShell. To get actual size under Per-Monitor DPI environment, run this method in PowerShell ISE.

MonitorHelper.ps1

Add-Type @"
using System;
using System.Runtime.InteropServices;

public static class MonitorHelper
{
  [DllImport("User32.dll")]
  public static extern IntPtr MonitorFromWindow(
    IntPtr hwnd,
    int dwFlags);

  [DllImport("User32.dll")]
  [return: MarshalAs(UnmanagedType.Bool)]
  public static extern bool GetMonitorInfo(
    IntPtr hMonitor,
    ref MONITORINFO lpmi);

  [StructLayout(LayoutKind.Sequential)]
  public struct MONITORINFO
  {
    public int cbSize;
    public RECT rcMonitor;
    public RECT rcWork;
    public int dwFlags;
  }

  [StructLayout(LayoutKind.Sequential)]
  public struct RECT
  {
    public int left;
    public int top;
    public int right;
    public int bottom;
  }

  public static bool GetPrimaryMonitorSize(out int width, out int height)
  {
    var handle = MonitorFromWindow(IntPtr.Zero, 1);
    var info = new MONITORINFO { cbSize = Marshal.SizeOf<MONITORINFO>() };
    if (GetMonitorInfo(handle, ref info))
    {
      var rect = info.rcMonitor;
      width = rect.right - rect.left;
      height = rect.bottom - rect.top;
      return true;
    }
    width = 0;
    height = 0;
    return false;
  }
}
"@

# To get actual size under Per-Monitor DPI environment, run this method in PowerShell ISE.
[int] $width = 0;
[int] $height = 0;
[MonitorHelper]::GetPrimaryMonitorSize([ref] $width, [ref] $height)
$width
$height
Pause