Hi Corey!
Layout is definitely a complex topic, often without one single right answer.
When it comes to font sizes in Rise 360 the safest option is often to stick with the pre-set sizes. I regularly tell people that it actually takes work to make a Rise 360 course look awkward because the default settings take care of a lot of the hard layout choices for you.
If you do want to change them, keep it simple and associate a font size with a specific kind of content. So maybe your body text is the smallest size, your sub-headers are all a consistent middle size, and your headers are all the same large size. Just make sure those size jumps are large enough to be easily noticed and there aren't too many of them.
As for using different blocks together, I've rarely seen an instance where two different blocks look terrible back-to-back. But sometimes if they're both a bit detailed your gut instincts might feel the overall lesson layout looks a bit busy. In cases like that, adding a bit of empty space (or whitespace) between the two blocks can help. An easy way to do that is to just add a blank Spacer Block from the Divider blocks option between them.
For more layout best practices, the articles we have on user interface and graphic design may be just what you're looking for. Here are a few to get started with:
Design Standards to Follow When Creating a Custom UI in Storyline 360:
https://community.articulate.com/articles/design-standards-to-follow-when-creating-a-custom-ui-in-storyline-3608 Questions to Ask When Choosing Fonts & Formatting Text:
https://community.articulate.com/series/community-team-articles/articles/8-questions-to-ask-when-choosing-fonts-formatting-text3 Essential Visual Design Concepts for E-Learning:
https://community.articulate.com/articles/3-essential-visual-design-concepts-for-e-learning