You would need to modify the theme files for that but I know others have done it using the child theme instead. The columns use span classes like “span3” or “span4”, so you would need to change them up or down. So if you wanted it smaller, you would turn span4 into span3, but you would also have to change the main content column as well. For example, the page with a right sidebar column has a layout like:
span8 for the main content and span4 for the sidebar. So if you take away from one, you need to add to the other, so then it would be: span9 and span3 (so that both side-by-side adds up to 12.