Feasible font size on the Retina MacBook Pro
It is impossible to deliver an "answer" to this question. You are asking for something that is purely opinion-based. I have friends who use super small fonts and it's very comfortable for them. I also have friends who use larger fonts, and that works best for them. I personally use a variety of font sizes depending on what Mac I am on and what App I'm using. Your best best is to work with whatever font size suits you, despite what anybody else tells you.

There is no "one-size-fits-all" answer to your question.