The size guide content is just a custom page. Create a new page with Gutenberg and add a table with the sizes information.
To get the same style from the demo content select Stripes style. And in the Advanced section add the custom class
See both options in context
Global settings (Customizer)
The global settings applied to all products are Appearance > Customize > Single Product > Side Guide
Size Guide Page is the page created in the previous step.
There are 2 basic filters: Add to Variable Products Only or select specific categories.
Otherwise, apply to all products.
Per product settings
If you have more than one size guide you can link a different page to override the global settings.
Edit the product, scroll down to Extra Content and select another page in the Size Guide section