You can easily adjust widths of your entire blog and that of the sidebars of your Blogger/Blogspot blog without using any code. You can easily increase or decrease the widths by using the Blogger Template Designer.
How To Get Started
== > If you are still using the old Blogger dashboard, you can log in to your blogger dashboard and switch to the new blogger dashboard so you can easily follow this tutorial.
Log in to your Blogger dashboard
Click on "Template" (on the right hand side menu). You will see something similar to the picture below:
Click on the "Customize" button. This will open up the "Template Designer"
On the left menu, you will see "Adjust widths", Click on it.
You will now see where to adjust the width as seen in the picture below:
Drag to your desired widths. (You will be seeing the preview as you are doing it).
Once satisfied, click the "APPLY TO BLOG" button at the top right corner of the Template Designer.
You can always follow the steps anytime to adjust the widths.
NOTE: If you downloaded a third party template that is not compatible with template designer, you might see an error message: "Not applicable to this template". For this kind of template, you will need to adjust the css of the template via "Edit html". This is just one of the reasons why I prefer "Template Designer Compatible templates".
If your template is compatible but can't view or see the preview in the Template Designer, you might have to install adobe flash to your browser.
I hope this helps.....