This tutorial features the Feast Plugin, which contains theme enhancements.
All modern themes from Feast (version 4.0.0+) come with 16px as the standard body font size, for reasons outlined in the font size for food blogs article. This was done to bring the themes more in-line with modern web standards, and to resolve "text too small" errors in Google Search Console.
Even just one year later, we're seeing stories of people benefiting from further increasing the body font size to 18px, and in the case of a few fonts, up to 20px.
As with all changes, you should be designing primarily for mobile devices. Stop editing your site for desktop display - that's not what 80% of your visitors see. Make sure it looks good first and foremost on mobile.
Update: the body font size is now reflected in the block editor with the Feast Plugin's Block Editor Styling
Below is the same page with the body font at different sizes:
Why increase the body font size?
The primary reason you may want to increase the body font size is if you've selected a custom font that's too small to read. Different fonts display at different sizes - some smaller, some larger.
This setting makes it easy to specify a larger body font size, which stays in place between theme updates.
A growing percentage of the population is reaching the age where they require assistance with reading small text, which means that a growing percentage of your audience would benefit from a larger text size.
The net effect of this is that it will take more scrolling to get through the same amount of content, because the text takes up more screen space.
And this is fine.
At the Google Webmaster Conference in November 2019, one of the presenting engineers specifically pointed out that scrolling is an intuitive, natural part of the mobile experience, and that extended content is perfectly acceptable on mobile.
We'd never recommend making a change just for the sake of ad revenue (your visitor experience is always factor #1).
In this case though, the incentives align, so it's worth noting: a large body font size may lead to an increase in RPM (ad revenue).
Ad companies such as Mediavine have recommended increasing the font size because it leads to higher RPMs.
While the cause of this remains unclear, we can speculate that it leads to a lower on-screen ad density, which publishers are willing to pay more for, because each ad gets more attention.
Search Engine Optimization
While we have no direct evidence of this, we've heard anecdotally that people have seen increases in search engine traffic around the time that they increase the body font size.
This can theoretically be tied to user experience: by both making the text easier to read for a growing percentage of the audience, and decreasing the ad clutter, visitors have a better on-site experience.
Additionally, sites that have too high of an ad density and may have been penalized by search engines for this, may be nudged outside of the "penalty" by the decreased ad density.
However, this is muddled by the fact that there are usually multiple user-experience improvements being made after an audit. There is no direct evidence tying font size to search traffic (except in the case of a warning shown in Google Search Console for text being too small).
The text headers are sized based on a percentage of the body font size, as explained in the font size for food blogs post. This is done to visually distinguish the headers from the rest of the text and make the page more legible, and scannable.
We do not recommend changing these ratios, which means that your header sizes will also increase.
The increased heading size means that you have to use headings intelligently. Make sure to follow our Modern Guidelines for Page Headings.
To prevent allowing people to shoot themselves in the foot with ridiculously small or large text, we've restricted the body font size to 16px - 22px.
The only problem we expect people may encounter is from caching (WP Rocket, server, CDN, browser), or plugins like autoptimize. Make sure you've cleared your caches, and check your site in a private/incognito window.