I don’t think we have a setting for it, but you could override the default.
The most custom way would be to add this CSS to your theme
:root {
--base-font-size: 30px; //whichever size you want
}
Unfortunately, this type of customisation isn’t available on basic plan 
You can however, choose a predefined size in the admin settings (Admin > settings > user preference > default text size(near the bottom):
Hope this helps!
Cheers