how do I change default font and font size for calc

I’d like to change my default font and its size in calc spreadsheets. Can someone tell me how to do this?

There’s no setting ‘for Calc’ insofar, but you can create a template having assigned your font and size to the ÄDefault’ cell style mainly.
Save the template,
import it as an assigned template with the template manager (>File>Templates>Manage Templates),
for ‘Filter’= Spreadsheets
make it your ‘Standard’ by rightclick>Set As Default.