If you haven't defined a width for that column, then that's what you will get returned.... a value of -1 means "default", and the default is defined for the worksheet
If you've loaded this workbook from a file that contains a default value, then you can get it this way, otherwise if you get a -1 value again, then it's because your spreadsheet viewer is permitted to use whatever default it wants, and this may change from reader to reader or version to version
$colWidth = $objectSelectedWorksheet->getDefaultColumnDimension()->getWidth();