If you are not comfortable with editing css for your site, you might try using Full Page for your page selection instead of Right Sidebar. You can then select the "Layout" option of your "add to main" area to manually create your own adjustable width sidebar (save it as a preset once you have it tweaked to your needs)
Its a little sloppy, but should achieve what you want.

Thank you for your suggestion. I would be comfortable with trying css, but I don't have access to the main css. As far as I can gather from other posts, because I did a quick install through Hostgator I can't get access through the root route: sitename.org/root

A good way to experiment is with the browser developer console. Load up the page and then in the developer console you can see which css rules apply to which parts of the page and edit them. These changes will be lost when you next load the page, but in the mean time you can see what sort of adjustments to widths, paddings and margins will achieve what you want and learn a bit about css in the process.

Where do I find the browser development console?

have a look at the last section of:
http://www.concrete5.org/documentation/how-tos/editors/getting-help...

It gives an overview and links to training resources for the developer console.

Thanks for that. Yes, the browser development console is useful. According to the console the width dimensions of the sidebar don't appear to be available to change in the Greek Yogurt theme.

I have figured out how to access the main css now, as well. And I don't see the sidebar dimensions option there either.