Option Explicit

Private Declare Function SendMessage 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()
  Dim i As Integer

  With Combo1
    .AddItem "Ein kurzer Eintrag"
    For i = 1 To 3
      .AddItem "Längere Einträge sind meistens nicht gut lesbar"
    Next i
    .AddItem "Noch ein kurzer Eintrag"
    .ListIndex = 0
  End With

  Call AutoSizeComboBox(Combo1)
End Sub

Sub AutoSizeComboBox(c As ComboBox)
  Dim maxBreite As Long
  Dim listBreite As Long
  Dim i As Integer

  maxBreite = c.Width
  For i = 0 To c.ListCount - 1
    listBreite = Me.TextWidth(c.List(i))
    If listBreite > maxBreite Then maxBreite = listBreite
  Next i
  SendMessage c.hwnd, CB_SETDROPPEDWIDTH, maxBreite + 20, 0
End Sub