New Post: Where is the width of the columns defined for the Cph sample site?

I'm trying to simply alter the width of the "Aside" column used in the pages of the Cph sample website included with C1.

I've searched and explored all of the various features and settings but cannot find this.

I have a page defined as "3 Columns" and in the master page I can see the definitions for these but I cannot find where the widths are defined.

So I have two questions:

1. Where are these widths defined?
2. How would I find this by myself? must I open the site in Visual Studio or something?
   

Thanks

Hugh