Custom Width for the Modules Left and Right

You can now easily set the width of the modules Left and Right in the Joomla! Administrator. These modules can be found next to your main content body (see the Module Positions page for their location). So not only are these columns fully collapsible (which means that if there are no modules published in particular area, that module position will not appear), but you can also set a custom width for each of them individually.

A small column on the right

You just want to have one small column on the right? Just don't assign any modules to appear on position left and set the width for the right column to something like 150 pixels.