To format a paragraph click on some text in the paragraph. Then click the "p" or "span" html element in the list at the bottom of the html editor.
This will highlight the paragraph and you can select the font style and size. Do not try to highlight the entire text by clicking and dragging over the text. This will only set the font on one element, and there may be more than one paragraph in the text. Each paragraph should be formatted individually.
If a font is not selected, the font options at the top will say "font" and default font will show up on the email after its sent. The default font for the email recipient may be different than the default font in the email designer. This can cause the email to appear with different fonts than you intend, so be sure to format each paragraph individually.