When people ask me about my small font size I just say "I'm far-sighted and smaller fonts simulate distance for me." Whether it's true or not doesn't matter, they leave me alone. I am far-sighted and smaller fonts are more comfortable for me to read, whether it's because of my far-sightedness or not is up to an optometrist to explain. People are more apt to leave others alone about something if that something has to do with a perceived physical necessity (it's like they're too embarrassed to continue discussing it), so it's been a pretty effective response.

So far as my font settings: Dejavu Sans Mono at 8 - 10pt or Inconsolata at 8 - 10pt depending on the program.