When you select the command, the Text Size control is displayed on the Status Bar.
The Radio button group (Small, Medium, Large) controls the text size. The size of Medium text is set by Font Selection dialog. Small text is half that size, and Large is twice the Medium size.
The text size is scaled along with everything else on the layout so text is always the same relative size as other layout objects.
Note: loading fonts (or changing sizes) can take a noticeable amount of time.