FolderSize is an open-source program that runs as a service in Windows. In the background, it collects and stores the size of every folder on your hard drive.
In FAR, I use F3 a lot of folders to chekc their size, but FAR doesn't cache the results. Would anyone be interested in developing a plugin that would tie into the FolderSize service to get the size of folders as you browse (or use a hotkey)?