When I consulted the help pages a few seconds ago, I found out how to change the default font size for characters in my spreadsheet. Then I created a new sheet, by pressing the [+] button. (FYI - I enclose the character in [ ] to denote the button). This created a new sheet as expected. Unfortunately, it failed to use 12 for the font size. Furthermore, I am not sure if I should be posting this problem here or as a bug to be squashed elsewhere.
In follow-up, I shut down the program. Then I opened it up again. The default font size is still 10. Ideally. the help pages will also include known bugs like this one, and perhaps what we as a community plan on doing to squash them.