con guggol se ne trovano di cose!

codice:
Option Explicit
Private Declare Function SetWindowLong Lib "user32" _
  Alias "SetWindowLongA" (ByVal hwnd As Long, ByVal _
  nIndex As Long, ByVal dwNewLong As Long) As Long
 Private Declare Function GetWindowLong Lib "user32" _
  Alias "GetWindowLongA" (ByVal hwnd As Long, ByVal _
  nIndex As Long) As Long
Const WS_MINIMIZEBOX = &H20000
Const WS_MAXIMIZEBOX = &H10000
Const GWL_STYLE = (-16)
Private originalWidth As Single, originalHeight As Single

Private Sub MDIForm_Load()
SetWindowLong Me.hwnd, GWL_STYLE, GetWindowLong(Me.hwnd, GWL_STYLE) And Not (WS_MINIMIZEBOX) And Not (WS_MAXIMIZEBOX)
originalWidth = Me.Width
originalHeight = Me.Height
End Sub

Private Sub MDIForm_Resize()
Me.Width = originalWidth
Me.Height = originalHeight
End Sub