WordPress gives you the opportunity to sort your posts in ascending or descending order. There are many scripts that go beyond that to allow you to order your posts based on other factors. Order Posts by Word Count is a simple plugin that displays your posts based on their word count. That means you can use this to show your longer or shorter posts first.
The plugin lets you order your posts in ascending and descending fashion. Just choose the number of posts you want displayed in your sidebar and their list type. This script does the rest. It uses your existing CSS rules but you can always change that to customize the look of the widget.
If you need to order your posts based on word count, the Order Posts by Word Count plugin is must see.