Since the release to manufacturing of Exchange 2013, you have been waiting for our sizing and capacity planning guidance. This is the first official release of our guidance in this area, and updates to our TechNet content will follow in a future milestone. As we continue to learn more from our own internal deployments of Exchange 2013, as well as from customer feedback, you will see further updates to our sizing and capacity planning guidance in two forms: changes to the numbers mentioned in this document, as well as further guidance on specific areas not covered here. Let us know what you think we are missing and we will do our best to respond with better information over time.
Continue reading here: http://blogs.technet.com/b/exchange/archive/2013/05/06/ask-the-perf-guy-sizing-exchange-2013-deployments.aspx
Until later,
Henrik Walther