You can define overall theme width as well as widths of the left hand side and the right hand side columns with widgets. All of this is done very easily in the theme options. 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 any widgets published in ‘left’ and/or ‘right’ widget sidebars on certain or all pages, main content area will occupy their space.