Common practise is to use the browser’s default font-size and scale the website using em and rem units (among others).

E.g. Chrome and Firefox have a default font-size of 16px. This is a good start. If you would like to increase that font size on bigger screens (non-mobile) you can use a media query and scale it up based on screen size.

When prototyping I often start off with a small screen (portrait phone) as default with a font-size of 1rem and increase it using breakpoints such as these: https://getbootstrap.com/docs/4.5/layout/overview/#responsive-breakpoints

Example: Serif fonts have extra details which are easier to distinguish at a slightly higher font size. I set the base font-size at 1rem (16px), on tablets and up a font-size of 1.125rem (18px), on desktops we scale it up even further to 1.25rem (20px), etc. I would recommend not to set the body font-size too large, as some people with bad sight or sitting further from their monitor may have their browser set to zoom. Unless I use special typefaces I tend to stick with 16-24px for body font-size. Everything else on the design scales based on that.

Optionally you can set the body font smaller (e.g. 12px) in your print stylesheet if printing is desired.