Open the site template editor
To learn how to open the site template editor refer to the following article: How to edit your site design
Select the dropdown for Body Font


Preview and then Publish changes

Updated
To learn how to open the site template editor refer to the following article: How to edit your site design
To change the text size you would need to use CSS. Here is a link to a recipe that shows how to change text colors. You could replace the CSS that sets the color with CSS that sets the text size.
http://help.screensteps.com/m/customizing/l/356530-css-and-javascript-customizations#font-color
Is it possible to import our own custom fonts?
@Ben - As long as the font is hosted on a public web server it is. See our example on using Google Fonts: https://help.screensteps.com/a/1014946-use-a-custom-google-font-on-your-site
Is there any way to change the default font size for the "text" -type authored areas? It seems really small on the default.