This support page explains about using the built in typekit fonts interface in your dashboard.
The theme you have chosen is a "responsive width" theme. That means it adjusts for browser windows and monitors narrower than the max width set in the CSS so that it looks good on everything from a desktop monitor to a smart phone. This means that widening the post area is almost a waste. If I narrow my browser window, or have it set narrow, the post area is going to shrink in width. Try it yourself, widen and narrow your browser window and see what happens to your posting area, and your entire main page.
You can narrow the sidebar so that the posting area takes up a larger percentage of the overall area, but there is a limit to how narrow you can take the sidebar before it ruins the balance of the theme. And again, that change would be negated if I go to your site and have my browser set at, say, 800px wide.