12 points.

There.  Though that means nothing, since your screen resolution, screen brightness, and blur point interpretation will make for much more potential variance than a simple point size answer.

So what’s a better answer?  

Printed page!  A book (who still has those?!).   Regular book, white page, black text, average font size (about 12 pt).  That’s the best way to measure, and to initially work on active focus.