Using Atto, you can do it manually. Click on Show/hide advanced buttons and the last button on the bottom row. Use an inline style tag to make it work, for example:
This font is bigger
<span style="font-size:17.7pt;">This font is bigger</span>
AFAIK, the current limitations of Atto don't allow for an easy change in size. Of course you may choose to swap editors to TinyMCE, but I don't know how much longer that is going to be supported in Moodle.