The PST Holding Area is used by PST Migrations into Enterprise Vault, but how big should you make it? Of course there are a few schools of thought on this topic, so let's discuss:
As Big As You Can
The first sort of flippant answer that you might often hear is that you should make the holding area as big as you can. Multi-terabyte even, if possible. But that's not particular good, I don't think.. it just shows lack of thought and allows lax management of the area.
A Manageable Size
It is far better to do some investigation work and try to figure out how big the holding area needs to be, and discuss with the backup team how big it can be so that they can comfortably backup the area each evening. If you use the Locate, Collect, Migrate for PST Migration then you can do the 'Locate' stage and get a good idea of the sizes of PST involved, and you will already have knowledge of the locations of end-users in terms of office, country etc. You could even use third party tools like PST FlightDeck from QUADROtech to do much more detailed and intelligent reporting and management of the 'flow' of data through a PST migration.
What you decide for the holding area size, the critical thing is that the location should be separate from anything else. This way if it really does run out of disk space it is not going to have a negative effect on the running of the Enterprise Vault server. Many people also use network attached storage, rather than a share on a locally attached drive on the Enterprise Vault server.
How do you size your PST Holding Area? Let me know in the comments.