Has a lot more to do with the amount of traffic you expect to handle.
Because hdd tend to come in fixed sizes, you don't typically have a lot of
choices in sizing when you take performance into account unless you're using
a SAN and really scaling the size of your Exchange server.

A couple of 72 GB drives mirrored for a SG's log files tends to work just
fine in many situations.

If you need to scale this, you'll want to figure out what the amount of
traffic you expect to handle, how large the average message is expected to
be, and how long you expect to be able to function if your backups fail to
execute (how long your log files should be able to continue before filling
the drive and shutting down the store).

Al