You'll have to modify some stuff in the main.css file (located in theme_clearview/themes/clearview/main.css). First, you'll have to modify the whole width of the #wrapper div (this div wraps all the content and sidebar). Then, you'll need to modify the width of #content and #right_sidebar #content, #left_sidebar #content, #page #content, #blog_entry #content, #pb_post #content (and #sidebar if you want to modify the sidebar's width) to whatever you want them to be.

I would recommend using Firebug for Firefox to inspect those elements/divs and make the modifications you need right in the browser via Firebug, then you can modify the main.css accordingly.

Let me know if you have any additional questions