Well, we just take some average recommendation depending on how many users you are planning to have on line at the same time and wohaa there is our server size guide for Moodle

I'm about to go on line next January with my Moodle server and the specs for it will be an Intel Xeon 2.66->, 2 Gigs of RAM, RAID 5 of course, hot spare about everything. Hardware is cheap. This is for round 120 simultaneous users.

Of course, there must be some room for scaling so, ah, we calculate the weighted average for every scenario