Windows 7 Explorer: how to show total size of all files in current folder?
http://classicshell.sourceforge.net/
Apparently there isn't an easy way, which is why the above freeware program was developed
If you navigate up one level, and right-click and select "Properties..." of the folder, it will tell you the total file size of the folder. You can also right click the background of the folder (not on a file) when you are browsing it and do the same thing.
However, this will also include the size of all subfolders.