Twenty Eleven – Padding Problem?

I’ve just recently installed WordPress and use the Twenty Eleven theme and want to maximise the width of the right side bar. The width of the sidebar seems to be around 300 pixels (for example) but when I insert text into the sidebar using a widget, it only seems to have about 200 pixels width.

Ideally, I would like the text to fill out most of the width if possible.

If anyone has any suggestions that would be really appreciated!

I have already created a child theme so just need to know which part of the system I need to edit/change.