Exchange 2003 disk space requirements per drive

I have a GroupWise environment and have been asked put on a project to migrate to Exchange 2003. I cannot seem to find any information on disk space requirements for the database and transaction log sizing for any size environment.

I have 2500 users and we average 30 inbound and 20 outbound per user per day. We use SAN so disk space is not an issue I just need to know how many GB to allocate for each drive.

Thanks for any help.