Since I wondered the same thing a while back and had to ask someone else how to find the setting to change it, I thought I'd share the instructions here with all of you, in case you'd also like to know. So, here you go....
How to Find and Use an Old Template in Blogger
A little disclaimer: I am hardly an expert in this sort of thing.... nevertheless, I hope this is helpful.
First, go to your blogger dashboard, and click on the "Design" tab.
Click on "Edit HTML" in the bar right under the big tabs along the top.
Before going any further, back up your current template, just in case you mess something up. You can back up your template by clicking on the "Download Full Template" link near the top of the page.
Ok, now that you've backed up your template, scroll down until you see the heading for "Old Templates."
Click on the link that says, "Select Layout Template." You will then see all of the old templates that used to be available in blogger before they made all the new changes.
In the lefthand column, go down to the second image, and you should see an option for "Minima Stretch." That's the template I'm using...
Check the circle next to the template you'd like to use. You can preview it if you'd like. Then click on "Save Template" near the top of the page.
You may have to change your banner width or adjust the sizes of images in your sidebar so that they also "stretch" out the full width of the available space you now have on your blog.
So there you go....
I hope this has been helpful. I'm by no means an expert, but if you have any other questions, I'll do my best to answer them, or find the answer if I can. Happy Blogging!