How to change font size in a Text Box

Website design generally benefits from having a limited number of font sizes for consistency. I would suggest that you set up the paragraphs in the fonts menu to be the size you want most of the time for Paragraph 2 and then use Paragraph 1 and Paragraph 2 set to two different, a little smaller or larger, to accommodate other uses.

You can set paragraph size manually in CSS or in a code block if you're willing to write CSS or HTML.