setting max-width of center column

Hello,

Firstly thank you for the amazing theme. I was going to make one from scratch for my new site but when I find Atahualpa decided to play around with it. So far it seems it's the perfect solution. I love how so much of the theme customization is doable in the WP-admin panel. Brilliant!

On to my query:

I am trying to find a way to set a max-width property for the center column. In Atahualpa's center column style max-width has no effect.

The reason I want this is that on some of my pages I have disabled the sidebars. I just want to set the center column to expand to no larger than 600px.

Also once I have achieved this I want to center the center column in the page (possibly with margin-right and left set to auto?).

Thanks you,
willowtree