I like to have as many lines of source code visible on my notebook or desktop monitor. And I think I have found a good balance between font size and readability.
On the other side: I’m getting older and my eyes are not getting any better. At the same time I noticed that students start using these ‘high-resolution-retina-displays’. They are great, but result in tiny default system fonts, so I have a hard time to read the source code on their machines.
Another challenge I noticed are the high-resolution projectors in class rooms or conferences. They are not well suited to show source code or text files because of the tiny fonts. Starting with Eclipse Neon there is an awesome feature which I can use to dynamically increase and decrease the font size which solves that problem: