Should text be BIGGER
Or
Should text be smaller

What grade are you in?
Your question doesn't cover nearly all that it should, since big and small are relative, and there are many, many factors.
If I am putting an ad on a 100' x 20' billboard on top of a 20 story building, what does an argument for small text do for me?
If you are limiting your project to electronic screens, specifically those on phones, tablets, and computers, please say so!