How to decrease the width of the sidebar for a specific page? Is it possible to decrease the width of my right sidebar for any specific page only? If so, please shed some light on that.

I have created a basic page whose node id is 56. In that page I want my right sidebar region to occupy less width than it normally does. This can be done through CSS but I want to know if there is a way to do that through Drupal, either with coding or without it.
You could use drupal_add_css() to conditionally include a custom CSS file on particular pages.

You could also add a custom template in your themes templates folder. If you clear caches after including it, it will automatically be included as long as it follows the Drupal template naming conventions.