Can you clarify what you mean by "how big"? In terms of disk space you should find that any server you buy will have more than enough disk space. On our server, which has been running a long time without archiving courses (for reasons that aren't worth getting into here), we're under 15GB of data, of which 13GB is due to the over 350 courses we have defined.
As far as power, we're obviously dealing with more users than you. My guess is that for your setup any multicore or multiprocessor system with 2GB of memory or more would be have no trouble at all handling the load. Someone else can correct me on this if they have better data, of course.