public class FormatKbSizeConverter : IValueConverter {
    [DllImport("shlwapi.dll", CharSet = CharSet.Unicode)]
    private static extern long StrFormatByteSizeW(long qdw, [MarshalAs(UnmanagedType.LPWStr)] StringBuilder pszBuf,
        int cchBuf);

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
        var number = System.Convert.ToInt64(value);
        var sb = new StringBuilder(32);
        StrFormatByteSizeW(number, sb, sb.Capacity);
        return sb.ToString();
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) {
        return DependencyProperty.UnsetValue;
    }
}