First, let's make sure that we're talking about the same thing. You're talking about the boxes next to the large double quote, correct?
If that's true, then it looks to me like there's a twitter widget involved because the id of the first box is id="twitter-widget-0". But that doesn't matter because the CSS is setting a specific size for the box at 440px.
I tested the CSS I provided by using Firefox and Firebug. It does work. But, I determined where the current CSS originates. It's coming from: http://platform.twitter.com/embed/embed.css . This appears to be a CSS file from twitter. This CSS file is loaded last, so the specification in this file of max-width: 550px !important is applied. Because it's last, it overrides anything before it - like style.css.
I assume this file is being loaded automatically. If not, remove it from whatever is loading it. Other options: 1) Use another technique for adding your tweets; 2) Load a CSS file after embed.css that includes the CSS I provided.