The program subclasses to watch for WM_NOTIFY messages.

Private Sub Form_Load()
    ... code deleted ...

    ' Subclass to watch for EN_REQUESTRESIZE messages.
    SetWindowProc Me.hWnd

    ... code deleted ...
End Sub

Private Type nmhdr
  hwndFrom As Long
  idfrom As Long
  code As Long
End Type

Private Type RECT
    Left As Long
    Top As Long
    Right As Long
    Bottom As Long
End Type

Private Type REQSIZE
    nmhdr As nmhdr
    RECT As RECT
End Type

' Look for the WM_NOTIFY message with an
' EN_REQUESTRESIZE message.
Public Function NewWindowProc(ByVal hWnd As Long, ByVal msg _
    As Long, ByVal wParam As Long, ByVal lParam As Long) As _
    Long
    On Error Resume Next

    ' See if this is WM_NOTIFY
    If msg = WM_NOTIFY Then
        ' Copy lParam into the nmhdr structure,
        CopyMemory m_nmhdr, ByVal lParam, Len(m_nmhdr)

        ' See if this is an EN_REQUESTRESIZE.
        If m_nmhdr.code = EN_REQUESTRESIZE Then
            ' Copy lParam into the REQSIZE structure,
            CopyMemory m_ReqSize, ByVal lParam, _
                Len(m_ReqSize)

            ' Make the RichtextBox tall enough.
            SetWindowPos g_RchWnd, VBNullPtr, _
                0, 0, _
                g_RichWidth, m_ReqSize.RECT.Bottom - _
                    m_ReqSize.RECT.Top, _
                SWP_SHOWWINDOW Or SWP_NOMOVE
         End If
    End If ' If msg = WM_NOTIFY Then ...

    ' Call the original WindowProc.
    NewWindowProc = CallWindowProc(m_OldWindowProc, hWnd, _
        msg, wParam, lParam)
End Function

HOWTO: Resize RichTextbox Control with Endless Bottom
Bottomless Rich Edit Controls