Ok. Here we go.
First off - create a child theme so that any changes that you make (such as site width) will not get overwritten if or when the theme updates. This is really important.
To do this add in the plugin One-Click Child Theme. Then go to Appearance>Child Theme and follow the instructions. Then go back to Appearance>Themes and check that it has installed ok.
Next you're going to make the site and side bar width changes that you want. These must be done in the new child theme stylesheet which you will find at Appearance>Editor. Click on Stylesheet and check that it is the one for the Child Theme.
To change the width of the site you need to change the #container div. Put the following code in at the very bottom of the child theme stylesheet.
Next to change the width of the sidebar put the following code in right at the bottom of the child theme stylesheet
This will reduce the width of the sidebar to 240px. Change this value if you want it a different width.
Hope this helps.