BuddyPress.org

I would recommend this as a great place to start looking into child themes. Its not advisable to edit the default theme, since your changes could be overwritten if/when you update your buddypress plugin.

For a quick a dirty change however you’ll need to change some values in the default.css file which can be found in wp-content / plugins / buddypress / bp-themes / bp-default / _inc / css / default.css

Your looking for ‘font-size:’ under body and h1, h2, h3, h4, h5, h6.

Hope that helps.