I know this is an older post, but I found a simple solution to the font size problem that should help other people. I scoured the internet and had trouble finding an answer. It is a known issue to Squarespace support that Safari doesn't display font sizes correctly. My understanding is that it is because 7.1 uses em sizes instead of px. So you have to add your desired size in pixels to the CSS:
For example, for Header 1:
Repeat as needed for each of the types that are not displaying correctly (p, h2, h3, etc.).