Implementing a complete Exchange 2010 solution requires not only good planning, but also a good understanding of all the technical requirements and a good knowledge of the several Exchange Server roles.

The below link give out some rules of thumb in order to ease the process of planning the different Exchange Server 2010 roles of your messaging infrastructure.

http://www.msexchange.org/articles_tutorials/exchange-server-2010/management-administration/exchange-2010-sizing-cheat-sheet.html