When I change the font size to 12 under Tools -> Options... the change does not get saved.

The dialog box looks like this after I change the font size to 12 and before I click the OK button.



After I click the OK button and open the dialog box again the value changes back to 10.

Wrote

When I change the font size to 12 under Tools -

"Changes will take effect after restart".
In a Smath work sheet, each region supports font size at will, from the Menu.
Very handy to reduce/enlarge some function/results.

Wrote

After I click the OK button and open the dialog box again the value changes back to 10.

In your Smath version the note is missing
"Changes will take effect after Program restart".

A very old issue, I'll take a look if can be easily fixed... the setting however is applied and available since you press ok (talking about recent versions)