A safe bet is to use 16 pixels for body copy. The sizing of course depends on the context and how many other things are on the page, but when in doubt, try to make things bigger.
Designing Perfect Text Field: Clarity, Accessibility and User Effort
Nick Babich
1K38

Agreed.

And, of course, always ensure that your form is responsive so that if the user zooms in, then the size of the text increases.

Do not set an upper limit: some users need huge text.