@rapidbernardos58 - Fonts and all other aesthetic style aspects of the page are controlled by the theme. This is not a limitation, but a feature of RapidWeaver.
This means that you can design your content independent of the style. Then choose a theme, change your mind, modify the theme, etc… all without impacting the style of the site.
It also has a huge advantage for new users. Many RapidWeaver users aren’t technically savvy – they just want something that builds nice pages and takes the headache out of web design.
Selecting font families that will look consistent on iPhone, Android, Mac, and Windows – on several browsers, over dozens of version… that is not a simple task for a new user. Leaving those decisions in the capable hands of theme designers who live and breath CSS, Typefaces, and browser compatibility is a Good Thing™
As far as size – it’s quite variable. The size of the type will be different depending on the size of the device that’s using it. It’s best not to think of type-sizes on the web in the same way as you do for a printed document. Things have to stay flexible and fluid in order to work on so many devices.