To increase font-size in Xcode is a pain.
For Xcode 4.1
Still a huge pain. Poor UI design (But my mindset does not seem to match the mindset of software engineers that make 100+ character variable and method names. Enough of my complaining)
I'll modify a previous post for the current version.
In the source editor box there is a list of types of text that you may set the font for: Plain text Comments Documentation Comments . . .
Select any or all items from the source editor list and the name and size of the font for that particular text will show up in the 'Font' window below the 'Source Editor' window. (If you happen to skip highlighting one of these, you will be able to get to the Font Inspector and select new sizes, but will wonder why the changes you make are not being applied!)
Congratulations. Your may now read your code. Wasn't that painless?