I have tried for years to find a way to reset the font and font size. LIbreOffice wants to use Liberation Sans 10 pt. I have changed it by going through saving a template and designating it as default. When I then open a new calc sheet it opens as the font and size that I want but as soon as I select a cell it reverts it back to Liberation Sans 10 pt.
Try this:
In Calc, open the Styles and Formatting window and right click on Default to modify it. Make your changes. Then follow steps 2-9 in [Tutorial] Creating a new default template (View topic) • Apache OpenOffice Community Forum
That process worked for me in AOO.