How do I auto size columns through the Excel interop objects?

Add this at your TODO point:

aRange.Columns.AutoFit();

This method opens already created excel file, Autofit all columns of all sheets based on 3rd Row. As you can see Range is selected From "A3 to K3" in excel.

 public static void AutoFitExcelSheets()
    {
        Microsoft.Office.Interop.Excel.Application _excel = null;
        Microsoft.Office.Interop.Excel.Workbook excelWorkbook = null;
        try
        {
            string ExcelPath = ApplicationData.PATH_EXCEL_FILE;
            _excel = new Microsoft.Office.Interop.Excel.Application();
            _excel.Visible = false;
            object readOnly = false;
            object isVisible = true;
            object missing = System.Reflection.Missing.Value;

            excelWorkbook = _excel.Workbooks.Open(ExcelPath,
                   0, false, 5, "", "", false, Microsoft.Office.Interop.Excel.XlPlatform.xlWindows, "",
                   true, false, 0, true, false, false);
            Microsoft.Office.Interop.Excel.Sheets excelSheets = excelWorkbook.Worksheets;
            foreach (Microsoft.Office.Interop.Excel.Worksheet currentSheet in excelSheets)
            {
                string Name = currentSheet.Name;
                Microsoft.Office.Interop.Excel.Worksheet excelWorksheet = (Microsoft.Office.Interop.Excel.Worksheet)excelSheets.get_Item(Name);
                Microsoft.Office.Interop.Excel.Range excelCells =
(Microsoft.Office.Interop.Excel.Range)excelWorksheet.get_Range("A3", "K3");
                excelCells.Columns.AutoFit();
            }
        }
        catch (Exception ex)
        {
            ProjectLog.AddError("EXCEL ERROR: Can not AutoFit: " + ex.Message);
        }
        finally
        {
            excelWorkbook.Close(true, Type.Missing, Type.Missing);
            GC.Collect();
            GC.WaitForPendingFinalizers();
            releaseObject(excelWorkbook);
            releaseObject(_excel);
        }
    }

This might be too late but if you add

 worksheet.Columns.AutoFit();

or

 worksheet.Rows.AutoFit();

it also works.

Also there is

aRange.EntireColumn.AutoFit();

See What is the difference between Range.Columns and Range.EntireColumn.