Go to Appearances > Montezuma Options and read the sections on CSS Settings (so you understand about the 12 column CSS grid system that Montezuma is based on), then Main Templates, so you can understand how the pages are created.
Do you want to change the width of the sidebar for all of the pages and posts on your site, or just that one post? If you want to change the width on all pages, then you would edit the index.php, page.php, and single.php virtual files in the Main Templates section and change the class of the two different columns, content and sidebar. The default is col8 for the content area and col4 for the sidebar. Just make sure your column classes add up to 12. So you can try setting content to col9 and sidebar to col3, for example.
If you want to change the sidebar for just that one post (and leave the sidebar the same on all other pages & posts), you would create a new Custom Page Template (Add main template) by copying single.php and giving it a name like narrow_sidebar, for example. You would then adjust the classes of the two column DIVs in your new narrow_sidebar.php virtual template: change col8 for the content and col4 for the sidebar to different values, as long as they add up to 12. Then go back and edit your post, and select narrow_sidebar as the Virtual Template for that post (scroll down and it will be towards the bottom right of your post editor).
