Hi,
Please edit templates/your_template/css/template.css and find div#ja-col1, you must see width:20%; (this is approx 240px I think) defined for div#ja-col1, you need to change the percentage. Since we are increasing column width, we need to reduce the main area width, so find #ja-contentwrap which has width:80% and reduce it by the same percentage you increase column width.
I am using full width template, so I need to change 20% – 80% values but in your case it may be different due to template width you are using.
If you can post your site url in forum, I can guide you better.