MacOS – Increase font size in OS

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.

It seems that there is are others that report a problem with font rendering in the iBooks app when using epub files under OS X Mavericks (10.9.x). As in your experience, the problem seems isolated to Macs that don't have a Retina Display, such as your Mac Pro.

The solution is fortunately a simple one: Go to

System Preferences > General

and uncheck the box that says Use LCD font smoothing when available.

This should clear up the jagged text you've been experiencing in iBooks on your Mac Pro.

(Note: You may need to quit and restart iBooks and/or restart your Mac for these changes to take effect.)