Use the Scale By box to specify a scaling factor that should be used to enlarge or shrink font size of the selected text.
Values less than 100% shrink font size. Values greater than 100% enlarge font size.
Example. Let's suppose that the selected text is formatted with 20 pts font size. If you have specified 150% for the Scale By box, the selected text will be formatted with 30 pts font size (30 is 150% of 20).