Hi Matthias,
I'm not sure how easy it is to change the size of columns in an Artisteer skin as there may be needed css changes in several places to accomplish it. It is much easier if you set the column and layout widths while working in Artisteer before exporting a template.
As far as I know, .art-contentLayout .art-sidebar1 {} is left column, .art-contentLayout .art-sidebar2 {} is right column and center will be .art-contentLayout .art-content-narrow {} if there is content in left and right, if there is content in only 1 side it will be .art-contentLayout .art-content {}, if there is no content in the sides then we use .art-contentLayout .art-content-wide {}
Hope it helps,
Joe