How do I change the standard font size in my slides?

I have tried using the slide master, but that does not seem to work.  I have gone through the following process:

- View / Slide Master - then select the relevant layout when I want to change the font size from 14 to 12

- Click the box in that layout, and change the font size

- Slide Master / Close Master view

But the font size in the slides which have that layout are unchanged.  So I

- right click thumbnail of a relevant slide

- select layout - and see it is set to the standard layout which I have just amended.

But still the font size does not change.

Same point applies to lots of other attempts to use slide master, e.g. to change font colour or adopt a certain quick style for a whole set of slides.  It just does not work.

What am I doing wrong?