Программа window_resizer.frm:

Private Declare Function SetWindowPos Lib "user32" (ByVal hwnd As Long, ByVal hWndInsertAfter As Long, ByVal x As Long, 
                                                    ByVal y As Long, ByVal cx As Long, ByVal cy As Long, ByVal wFlags As Long) As Long
Private Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hwnd As Long, ByVal lpString As String, 
                                                    yVal cch As Long) As Long
Private Const SWP_NOMOVE = &H2
Private Const HWND_TOPMOST = -1

'Конвертируем из HEX в длинное число, нужно для HWND
Function Hex2Dbl(h As String) As Double
    Hex2Dbl = CDbl("&h0" & h)
    If Hex2Dbl < 0 Then Hex2Dbl = Hex2Dbl + 4294967296#
End Function

Private Sub Command1_Click()
  'Меняем размер окна
  SetWindowPos CLng(Hex2Dbl(Me.Text3.Text)), HWND_TOPMOST, 0, 0, CLng(Me.Text1.Text), CLng(Me.Text2.Text), SWP_NOMOVE
End Sub

Так выглядет форма: