how can I change default cell sizes in Libre Office Calc
Size values aren’t a property of cells, but of colums (width) and rows (height). Since there are neither column styles nor row styles, there also are no defaults you can change.
You only can create templates for spreadsheets documents containing sheets having all the properties (including some contents if wanted) set as you like it - depending on the purpose of each template.
The actually used sheets can then be derived from the original ones or from copies you made of them.
The short answer: The defaults for new sheets are hard-coded.
Hi @Lupp, something can be done with the height, changing default font size, for a size larger than 10 points.
Hmmm…
Despite the fact that there aren’t (and cannot reasonably be) columnn/row styles, the “hard-coded” values (2258, 452) under discussion here could be made editable via the “Expert Configuration”.
A sheet also could have set (and save) user defined default values for the purpose.