A good place to start for sizing Microsoft AD, Exchange, MSSQL, System Center, Forefront and Sharepoint, would be the technet.
Windows Server
Exchange Server
MS SQL
System Center
ForeFront
SharePoint
The documentation includes deployment planning, architecture design, h/w sizing, virtualized environment, but you actually have to spend some time to absorb the info, and some imagination to visualize it (there isn't much images to support the documentations)
