No, font size does not affect to SEO, but to make an attractive and eye-catching website font size does matter. Also, it is a good practice that helps to grab your visitor’s attention on the website. Furthermore, playing with font colors and font size has become a very trendy process.
Though font size does not matter in SEO, search engine can track user interaction with the website pages. If the users bounce back due to unattractive font size, search engine can consider it as inappropriate for the keyword that it is used search for.
This is very true. Using the default page font size and using em or rem modifications is the most appropriate approach and will translate well to mobile devices. Font size doesn't have a huge impact on SEO (aside from mobile readability), but your usage of H1 tags, bold and other headers will have an impact.