cpjust,
Are you sure you're waiting long enough? If the folder has LOTS and LOTS of files, then it will take Explorer a long time (some times) to return a number in that spot.
Also, have you tried making the Status Bar is enabled in Explorer, then simply select that folder in explorer view (when it's on the right, by default), and it'll show in the status bar.
But still, you have to wait long enough.
If you want to see the folder size for more than one at a time, there are 3rd party applications for that.
Also, if you're having some other issue that you can't sort out, and just need to see the file size for a certain folder, try an FTP program, such as a new one I'm using now,
BitKinex, or try something like
FreeCommander as an alternative to Windows Explorer.