Folder Size (in folder tree)

Could we get an option to display folder sizes in the folder tree?

Too often I find myself needing to know the size of a given folder, for example when trying to find out if it's contents will fit on a CD or DVD, while shuffling around files and folders. It would be excellent if I didn't have to open that folder or the parent of that folder, in a lister, to get the size.

You could always just do Properties on the folder?