Changing width in Bartik theme
If you're not committed to using Bartik, you could try one of the Adaptive Theme subthemes.  These themes are very popular (one of them, Corolla, was intended for inclusion as the default theme in Drupal 7 but lost out to Bartik) and they allow you to change the width of the page easily on the settings page.

They are also responsive, which means that they will automatically adjust the columns on your page to display better on different screen sizes (by default, they provide PC, tablet, and smartphone layouts).  You can also adjust the colors from within the UI just like Bartik.  They are very newbie-friendly (the other major collection of newbie-friendly Drupal themes are the Fusion subthemes).

The base theme is Adaptivetheme, which you must install (though you won't actually use this theme).  Then you can choose from Corolla, Sky, or Pixture Reloaded.