Bullit is the style I developed to read my RSS feeds on NetNewsWire. Clarity and readability were the most important factors. Bullit uses 70% (and up to 700px) width for the reading column. Body copy is 13px Lucida Grande.
I’ve installed it and have to say it really is a beautiful theme. Makes reading in my feed reader a little more tempting, it’s gonna be a challenge to get me to read it on your site now. If your designs is good enough it will happen. bit of a challenge for you.