It sounds like a simple question: what font size to use for mobile website? However, it seems like a lot of mobile content these days is aimed at fitting several different screens, but rarely you see font sizes being optimized for the mobile experience. Whatever happened to readability? In the image featured in this article, you can clearly see the difference between font sizes.
Keeping in mind that not everyone in your audience has perfect 20/20 vision, you should plan for a reading experience that covers a wide spectrum. In my opinion, a mobile website with extremely small fonts is the equivalent of sending mobile optimized e-mails to your customers only to re-direct them to a non-responsive website.
If you want to optimize the reading experience for your website visitors, but don’t want to make the current font-size for your widescreen version any bigger, you’ll have to get your hands dirty with some CSS media queries. This website does not not use media queries, as the font size for the desktop version is big enough that it is also very easy to read on a small screen (line spacing goes a long way!).
Want to find out how to use media queries on your website? Trent Walton has been experimenting with them for awhile. Here’s a good article he posted on the subject last year (2012).