Option Explicit

Private Const WM_GETMINMAXINFO = &H24&
Private Const GWL_WNDPROC = (-4)

Private Type POINTAPI
  X As Long
  Y As Long
End Type

Private Type MINMAXINFO
  ptReserved      As POINTAPI
  ptMaxSize       As POINTAPI
  ptMaxPosition   As POINTAPI
  ptMinTrackSize  As POINTAPI
  ptMaxTrackSize  As POINTAPI
End Type

Private Declare Function SetWindowLong Lib "user32" Alias _
      "SetWindowLongA" (ByVal hWnd As Long, ByVal nIndex _
      As Long, ByVal dwNewLong As Long) As Long

Private Declare Sub CopyMemory Lib "kernel32" Alias _
      "RtlMoveMemory" (ByRef lDestination As Any, ByRef _
      pSource As Any, ByVal Length As Long)

Private Declare Function CallWindowProc Lib "user32" Alias _
      "CallWindowProcA" (ByVal lpPrevWndFunc As Long, ByVal _
      hWnd As Long, ByVal uMsg As Long, ByVal wParam As Long, _
      ByVal lParam As Long) As Long

Private m_PrevWindowProc As Long

Private m_MinWidth  As Long
Private m_MinHeight As Long
Private m_MaxWidth  As Long
Private m_MaxHeight As Long

Public Sub LimitWindowSize(ByRef Frm As Form, _
      ByVal MinWidth As Long, ByVal MinHeight As Long, _
      ByVal MaxWidth As Long, ByVal MaxHeight As Long)

  If m_PrevWindowProc <> 0 Then
    ReleaseWindowSize Frm
  End If

  m_MinWidth = MinWidth
  m_MinHeight = MinHeight
  m_MaxWidth = MaxWidth
  m_MaxHeight = MaxHeight

  m_PrevWindowProc = SetWindowLong(Frm.hWnd, GWL_WNDPROC, _
             AddressOf WindowProc)
End Sub

Public Sub ReleaseWindowSize(ByVal Frm As Form)
  If m_PrevWindowProc <> 0 Then
    SetWindowLong Frm.hWnd, GWL_WNDPROC, m_PrevWindowProc
    m_PrevWindowProc = 0
  End If
End Sub

Private Function WindowProc(ByVal hWnd As Long, ByVal uMsg As _
    Long, ByVal wParam As Long, ByVal lParam As Long) As Long

   Dim MMI As MINMAXINFO

   Select Case uMsg
      Case WM_GETMINMAXINFO
        CopyMemory MMI, ByVal lParam, Len(MMI)

        With MMI
          .ptMinTrackSize.X = m_MinWidth
          .ptMinTrackSize.Y = m_MinHeight
          .ptMaxTrackSize.X = m_MaxWidth
          .ptMaxTrackSize.Y = m_MaxHeight
        End With

        CopyMemory ByVal lParam, MMI, Len(MMI)
        WindowProc = 0

      Case Else
        WindowProc = CallWindowProc(m_PrevWindowProc, _
                hWnd, uMsg, wParam, lParam)
   End Select
End Function

Option Explicit

Private m_FrmWidth  As Long
Private m_FrmHeight As Long

Private Sub Form_Load()
  m_FrmWidth = Me.Width
  m_FrmHeight = Me.Height
End Sub

Private Sub cmdLimitWindowSize_Click()
  Dim lngMinHeight As Long
  Dim lngMinWidth  As Long
  Dim lngMaxHeight As Long
  Dim lngMaxWidth  As Long

  lngMinWidth = Val(txtMinWidth.Text)
  If lngMinWidth <= 0 Then
    lngMinWidth = m_FrmWidth
  End If

  lngMinHeight = Val(txtMinHeight.Text)
  If lngMinHeight <= 0 Then
    lngMinHeight = m_FrmHeight
  End If

  lngMaxWidth = Val(txtMaxWidth.Text)
  If lngMaxWidth <= 0 Then
    lngMaxWidth = Screen.Width / Screen.TwipsPerPixelX
  Else
    If lngMaxWidth < lngMinWidth Then
      lngMaxWidth = lngMinWidth
    End If
  End If

  lngMaxHeight = Val(txtMaxHeight.Text)
  If lngMaxHeight <= 0 Then
    lngMaxHeight = Screen.Height / Screen.TwipsPerPixelY
  Else
    If lngMaxHeight < lngMinHeight Then
      lngMaxHeight = lngMinHeight
    End If
  End If

  Call LimitWindowSize(Me, lngMinWidth, lngMinHeight, _
            lngMaxWidth, lngMaxHeight)
End Sub

Private Sub cmdReleaseWindowSize_Click()
  ReleaseWindowSize Me
End Sub

Private Sub Form_Unload(Cancel As Integer)
  ReleaseWindowSize Me
End Sub