He encontrado este modo para hacerlo con TextBox normales.
Código :
Ver originalPrivate Declare Function SendMessageLong Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
Private Sub Text1_Change()
If SendMessageLong(Text1.hwnd, &HBA, 0&, 0&) > 5 Then
SendKeys "{BS}"
End If
End Sub
Saludos