I somehow have created a monster, a spreadsheet which has copied my 100 rows/20 columns of data and repeated them downward below my original data until there are over a million rows. Can I select an unused row below my data, then clear/delete everything below it? As it is, I can’t use this spreadsheet because it takes several minutes to load and to save. This is very important data and I have no backups saved. Thanks.
If the relevant data is just 100 rows, the easiest way would be to select these 100 rows (two clicks: on first and last row, 2nd one using Shift+Klick), then hit Control-C to copy them to clipboard, Control+N for new sheet, Control+V to paste.
Select a cell just below your last useful data; Ctrl+Shift+ArrowDown; menu Sheet → Delete Rows.
Save As with a new name to be able to return to your old date in case you see later that something is lost.