You can change the sidebar width by targeting its element with some custom CSS:
#secondary {
width: NNNpx;
}
Change NNN to your desired width.
If you’re trying to make the column wider than it is now, you’ll need to make the overall layout wide enough to accommodate it. The main container div is #content.
Don’t edit the theme files directly, otherwise your changes will be overwritten whenever the theme is updated.
Since you’re already using Jetpack, an easy way to add custom CSS is to activate the Custom CSS module. You’ll then add your custom CSS in the new stylesheet editor that’ll appear in your dashboard, under Appearance > Edit CSS.
You’ll want to be very careful with these sorts of layout changes, as they can affect the mobile display. You may need to add new media queries to make sure the site still displays correctly at small screen sizes.
You can learn more about using media queries that target certain screen sizes here:
http://en.support.wordpress.com/custom-design/custom-css-media-queries/
http://css-tricks.com/snippets/css/media-queries-for-standard-devices/
http://webdesignerwall.com/tutorials/responsive-design-with-css3-media-queries
