The changes are saved automatically when you exit the preferences dialog. You have the restart Lr after changing the Font to have them take effect. It tells you that at the bottom of the dialog when you make the change but it is very unreadable and easily missed and it is light gray letters on lighter gray background. BAD interface design.
Thank you for responding to my question. I should have included more information with my initial question.
I have LRC on 2 laptops. On one the interface font is 200% and the other is 150%. On both machines I've selected smaller font sizes, closed LRC, restarted the application and the same, larger than desired interface font sizes appeared. I've also made changes to the preferences, closed LRC, rebooted the machine, opened LRC again, with the same problem.
I don't know what I should do for the next step(s) but simply making a change on the preferences screen isn't working. Please advise.