My Windows Explorer in XP shows folder sizes automatically. When folders are shown in the Contents pane, all I need do is place my cursor over the folder and a pop up windows displays the folder size.
Like thius:
That only works in the Contents Pane.
And, if you need more info, right click on the folder, select PROPERTIES, and it will tell you the size, how many files in it, etc. This works in either the Navigation Pane or the Contents Pane.
And, it works exactly the same way in Vista and Windows 7.