zandyring on "How do I manage fonts and font size in an individual post or textbox?"

You can change your font body size here:
https://shorelineea.wordpress.com/wp-admin/themes.php?page=custom-fonts

It's a better practice to use CSS to manage all aspects of the look of the site (you're right - you shouldn't have to do it line by line!). If you want to only increase a few paragraphs, you can add a new class in your custom CSS style sheet, and use that.

To do that, you would want to define the class and the size. So maybe:

.bigparagraph {font-size: 16px;}
.smallparagraph {font-size: 12px;}

And adjust from there. You would need to add these classes to your HTML still, but at least you could target specific paragraphs as you see fit.

If you need more information, please just let me know!