For the impatient:
Download folderstat.cpp
Folderstat is a program that reports the sizes of each of the foldrs in an IMAP account. It uses pine as an IMAP client, so you'll need to have an IMAP account set up in pine, and you'll need to have pine in your path.

Unless you have pine set up to use ssh-agent or a password file, it will ask you to authenticate once for each folder, so it's not really useful for accounts that use SSL/TLS or otherwise require authentication. However, it is a decent example of iostreams and STL usage in C++.

To compile on *nix, just do g++ folderstat.cpp. On Windows, just load it in your IDE of choice and compile it. :P

Eventually, I might re-implement this using Python's imaplib, including support for authenticating to GSS-API capable imap servers.

Folderstat is Copyright 1999 Ryan Barrett, and is distributed under the GPL.