Private 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 Const CB_SETDROPPEDWIDTH = &H160

Private Sub Form_Load()
'Число '180' устанавливает ширину ниспадающей части: замените его любым вашим числом
r = SendMessageLong(Combo1.hwnd, CB_SETDROPPEDWIDTH, 180, 0)
End Sub