Exchange Disk Sizing

Here are some articles about disk sizing and general sizing for Exchange.

http://www.msexchange.org/tutorials/Art-Science-Sizing-Exchange2003-Part1.html
http://www.msexchange.org/tutorials/Art-Science-Sizing-Exchange-2003-Part2.html
http://blogs.technet.com/exchange/archive/2004/10/11/240868.aspx
http://blogs.technet.com/evand/archive/2004/10/14/242127.aspx
http://blogs.technet.com/exchange/archive/2004/11/03/251743.aspx

and a real world example

http://msmvps.com/ehlo/archive/2005/10/18/71487.aspx