In order to change your column widths you will first need to create a custom style. Create a copy of the style you are working on, or click "Copy Style" when prompted on the Banner & Navigation page.
Step 1 - Edit Style
Switch to the Style Mode in the upper right corner of your screen.
Step 2 - Edit Widths
Navigate to the Banner & Navigation tab in Style Mode and select Change Widths.
Step 3 - Select Area
Choose the area on your site you would like to adjust from the drop-down menu.
Step 4 - Change Width
Use the slider to adjust the width, or enter the value for this in the Current Value field. Be sure to click to Save Changes to apply this to your style.