You can define overall template width as well as widths of the left hand side and the right hand side columns with modules. All of this is done very easily in the template configuration. Main content area will be calculated to fit best between the left hand side and the right hand side column, no matter what values you entered for them. Also, if you don't have left or right modules published on certain pages, main content area will occupy their space.
Image shown above is from another template. However, this template has the same feature.