First, click Design in the top right corner of your blog. That brings you to this page (or one similar) if you are using the newest version of Blogger:
If you aren't using the newest version of Blogger and want to do this, just click that line at the top that says something like "Try Blogger's new look". Afterward, if you don't like it, you can always "switch back" just by clicking those words (found by my profile pic in the upper right). I switch back and forth a lot since you can't do everything in either format.
Now see that orange button that says "Customize"? It's right below the picture of my blog. Next you click that. Now you see something like this:
Not all templates have all the same options, but this is where you can make all the tweaks to your design. I particularly like to go to "Advanced". You can mess with tons of your design there. So much fun!
Anyway, to adjust your widths, click on (duh) "Adjust widths". Now you get:
Here you can move the slider or type in numbers until you get the look you want. Previously I adjusted my sidebar here as well so that some of the things I had over there would fit better. This last time I just moved the slider until pictures fit how I wanted.
When you get the look you want, don't forget to hit that orange button in the top right that says "Apply to blog".
Simple, right? Any questions? Let me know in comments and I'll try to help you out :)
