Every time I open a file with Calc, no matter how many columns it has, by default I get for free a ludicrous amount of columns and rows (eg: till col AML and row 100.000)
I can't find the configuration for this. I'm on macOS Monterrey and the LibreOffice version is 7.3.6.2
You can select the columns and rows that you dont want and then hide the rest.
Select the columns you want to hide. Click on the last column header that you want and then hit Ctrl+Shift+Right Arrow. Then right click on any header that is selected and left click on Hide Columns.
Select the rows you want to hide. Click on the last row header that you want, and then hit Ctrl+Shift+Down Arrow. Then right click on any header that is selected and left click on Hide Row.
If you need to add Columns or Rows after doing the above, then make sure to Insert Row or Column before the last row or column.
Once you have the sheet setup the way you want it. Then click on File | Templates | Save as Template. Name the template something like My Default, and make sure to check 'Set as default template' at the bottom of the Save as dialog.
Now, when you open Libre Calc you will have your 'smaller' spreadsheet. (And setting fonts, or zoom etc will also be saved this way.)