The sizing information for Exchange 2013 was released much earlier this year, but as I have just been asked for it I thought that I might as well copy the guidance into a blog for future re-use!
– Use Exchange 2013 pre-requisites to determine what software is required for an Exchange 2013 on-prem implementation
– Consider co-locating CAS and MBX – CAS is now a very light stateless proxy with almost all load on the mbx server. See blog posts below
Of course if you choose Exchange Online you don’t need to worry about any of the above – Microsoft do that for you!!