You can find the content width options under Site Options > Layout > Global.
There are 3 options for the width: Narrow (760px), Wide (1200px) and Full width (full width of the container).
You will also find the display options for your sidebar: on the left, on the right, or no sidebar.
Under Layout > Page, you will find similar settings for your container and your sidebar, which will apply to all pages.
Under Layout > Blog, you will find the same options to customize the appearance of your posts and category pages.
Under Layout > Homepage, you will find these same widths and sidebar settings, which will apply to the homepage only.
After selecting your global width settings, you have the option to override the width you have selected at the individual page level or post level in Layout settings.
Also, some blocks are able to override the global or page settings. Currently, those blocks are:
The featured post
The latest posts block
The about us block
The progress bar
This is useful if you would like most of your post or page to follow a set width, but have one of your blocks be wider or narrower than the rest of the content.