You can use the AfterColPosChanged event to get the new width of the column:

private void ultraGrid1_AfterColPosChanged(object sender, Infragistics.Win.UltraWinGrid.AfterColPosChangedEventArgs e)
{
    if (e.PosChanged == Infragistics.Win.UltraWinGrid.PosChanged.Sized)
    {
        UltraGridColumn col = e.ColumnHeaders[0].Column;
        int newWidth = col.Width;

        // Do things with the width...
    }
}

-Matt