This is one of the easy fixes when it comes to Search Console Mobile Usability report showing Text Too Small to Read issue. Basically, there is a text Google sees that is below required specification (less than 16px font size will usually cause text too small to read problem).
Here is the CSS Rule for Fixing Text Too Small to Read
However, its better to use responsive font sizes optimizing for Core Web Vitals (part of Page Experience Update) like so:
All you have to do is identify which text element is too small on a web page.
Here’s a Video Tutorial That Shows How to Fix Mobile Usability Errors
Here’s What Google Search Console Says About Text too small to read
The font size for the web page is too small to be legible (legible for web means, readable). Any font (including menu, footer links etc.) that requires mobile visitors to “pinch to zoom” in order to read is considered as text too small to read.
After specifying a viewport meta tag for your web pages (that would look something like this in the code of your website)
<meta name="viewport" content="width=device-width, initial-scale=1" />
width=device-width basically means width should respond to whatever the device width is (mobile phone portrait orientation let’s say 400px on Galaxy smartphone, or 678px width in iPad for example) the viewable area should resize to.
initial-scale=1 basically means, zoom 100% to whatever the viewport dimension is according to mobile device. The initial-scale property controls the zoom level when the page is first loaded. Its to do with how users are allowed to zoom the page in or out.
All of the above means set your font sizes to scale properly within the viewport because the viewport dimensions change according to your website visitors devices being used to visit your web pages.
Learn more about using Legible Font Sizes in responsive web design basics to avoid text being small