Public Class Form1
    Dim str As String = "abcdefghijklmnopqrstuvwxyz"

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        ListBox1.Items.Add(TrimToSize("abcdefghijklmnopqrstuvwxyz"))
        ListBox1.Items.Add(TrimToSize("This is a major line"))
        ListBox1.Items.Add(TrimToSize("Thomas, is that you?"))
        ListBox1.Items.Add(TrimToSize("It's alive!"))
    End Sub

    Private Function TrimToSize(ByVal Str As String) As String
        Dim wid As Integer = ListBox1.ClientSize.Width
        Dim g As Graphics = ListBox1.CreateGraphics
        Dim siz As New SizeF
        Do
            siz = g.MeasureString(Str, ListBox1.Font)
            If siz.Width > wid Then Str = Str.Substring(0, Str.Length - 1)
        Loop While siz.Width > wid
        g.Dispose()
        Return Str
    End Function

End Class