Declare Function SendMessage Lib "user32" Alias
"SendMessageA" (ByVal hWnd As Long, ByVal wMsg As Long,
ByVal wParam As Long, lParam As Long) As Long

Const WM_USER = &H400
Const LB_SETHORIZONTALEXTENT = WM_USER + 21

Sub CheckListBox
    
dim
 i%, res&
    
dim
 Scrollwidth&
    
    For i% = 0 To Form1.List1.ListCount
       If Form1.TextWidth(Form1.List1.List(i%)) > Scrollwidth& Then _
        Scrollwidth& = Form1.TextWidth(Form1.List1.List(i%))
    Next i%

    res& = SendMessage(Form1.List1.hWnd, LB_SETHORIZONTALEXTENT, _
        Scrollwidth&, 0&)
End Sub

Call this 
procedure
 everytime the 
content
 of the listbox has changed.