I always wanted windows explorer to display the size of folders as it displays the size of files in details view.I came through an application that  adds a new Folder size column to your explorer.This column displays size of folders(Size of all files and folders in that particular folder) in addition to size of files.

Though application is available for Windows XP and windows 2000 only,but its really cool application that helps you in monitoring and cleaning of your folders.

You can Download Folder Size from sourceforge

How to use it