Private Const EM_LIMITTEXT          As Long = &HC5

Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
                                (ByVal hWnd As Long, ByVal wMsg As Long, _
                                 ByVal wParam As Long, lParam As Any) As Long

Private Sub Form_Load()

    (中略)

    With テキストボックス
        If .MaxLength <> 0 Then
            Call SendMessage(.hWnd, EM_LIMITTEXT, .MaxLength, 0)
            .MaxLength = 0
        End If
    End With

    (中略)

End Sub