The font typeface, styles, and even the size for text in your presentation are dictated by the active Theme or Theme Fonts set. You can certainly override these defaults, and select another font typeface and also change the font size. Beyond changing the typeface and size, you can also make a certain word or phrase stand apart by formatting it bold, italic, underlined, etc.
To learn more, choose your version of PowerPoint. If we do not have a tutorial for your version of PowerPoint, explore the version closest to the one you use.