I want to run SpamAssassin for my large site. What specification of mail server do I need?

There are so many different variables, it's very difficult to make suggestions. Some factors that will affect the answer are:

1. How many users are there?
   

2. Will they all be having their mail scanned?

3. How much mail does each user receive daily (number of messages, total size in bytes)?

4. Does mail arrive evenly throughout the day, or is it "bursty"?

5. Are you doing other processing on that server (POP3/IMAP mailboxes, virus scanning, ...)?

For very large sites, it's worth noting that a single huge machine may not be necessary: using multiple machines running spamd, with your MX on a separate machine, using a DNS round-robin record to load-balance, will probably work quite well.