TextBox has a MaxLength property.  Unfortunately, ComboBox doesn't.  Here is how to get around this problem.  To set the maximum length simply call SetComboBoxMaxLength with the handle of the parameter (Combo1.hWnd) and the MaxLength of your choice.  To remove the maximum length afterward, call SetComboBoxMaxLength with MaxLength parameter of 0.

Option Explicit

'general declarations
Private Const CB_LIMITTEXT& = &H141
Private Declare Function SendMessage Lib "USER32" Alias "SendMessageA" _
         (ByVal hWnd As Long, ByVal wMsg As Long, ByVal wParam As _
         Integer, ByVal lParam As Any) As Long

'call this procedure to set or remove the maximum lengths for the ComboBox
Public Sub SetComboBoxMaxLength(hWnd As Long, MaxLength As Long)
    SendMessage hWnd, CB_LIMITTEXT&, MaxLength, 0&
End Sub