Private Sub Text1_Change()
    If Len(Text1.text) > 20 Then
        Text1.text = Left(Text1.text, 20)
    End If
End Sub

Text1.MaxLength=20

Private Const CB_LIMITTEXT = &H141
Public Sub SetComboMaxLength(ComboBox As ComboBox, ByVal lMaxLength As Long)
    SendMessageLong ComboBox.hWnd, CB_LIMITTEXT, lMaxLength, ByVal 0&
End Sub

'SendMessageLong function
Public Declare Function SendMessageLong Lib "user32" Alias "SendMessageA" _
(ByVal hWnd As Long, _
ByVal Msg As Long, _
ByVal wParam As Long, _
lParam As Long) As Long