The Microsoft Exchange team has released the Exchange 2013 Server Role Requirements calculator.
From the MS Exchange Team blog:
It’s been a long road, but the initial release of the Exchange 2013 Server Role Requirements Calculator is here. No, that isn’t a mistake, the calculator has been rebranded. Yes, this is no longer a Mailbox server role calculator; this calculator includes recommendations on sizing Client Access servers too!
I recommend reading the rest of that post because it does outline some very interesting things about Exchange 2013 sizing.
Download the calculator here.