Blogger has added the ability to customize the widths of the sidebar and posts of the new Blogger themes via the Blogger Theme Designer.
To customize the widths, go into the Blogger Theme Designer, click “Advanced,” and there you will find a “Widths” section.
Video demo :
hooray!!! 
I wonder why they didn’t add that at first 
Am sure the guys are still working on better functionality for the template. Blogger platform is now becoming more interesting
This is cool. But it is not on my template yet.
Not on mine too
You’ve got to upgrade your theme.
Do I have to reinstall the template or how do I upgrade it so I can change the width size.
Yes, re-install but ensure you back up…
What of all the customized changes that have done on it. Will start from scratch again
Am seeing this late
I had to cheat late night sleep before i discovered it myself