Font Size - yihui/xaringan GitHub Wiki

You can change the font size of single words, sentences or complete slides by defining two new classes in my-theme.css which change the font size relative to its default. Here, they are named .small and .large:

.large { font-size: 130% }
.small { font-size: 70% }

Simply enclose the text you want to change in your Presentation in .small[some text here] or .large[].
Of course, you can change the percentages of this example to your desires or define more custom classes with other values like .tiny{font-size: 40%}.