In general, separate your MTA services into inbound and outbound services. You can then size each in a similar fashion. The goal of sizing your MTAs is to determine the maximum number of messages that can be relayed per second.
From the raw performance of the inbound MTA, add SSL, virus scanning processes, and other extraordinary message processing.
With redundancy, one or more of each type of machine can still handle peak load without a substantial impact to throughput or response time.
In addition, sufficient disk capacity for network problems or non-functioning remote MTAs must be calculated for transient messages.