totally depends on what the service running are and how the user configures them. we really can't comment on an "ideal" size, there's no such thing. often /tmp will be a tmpfs / shmfs filesystem which is actually held in ram (although potentially swapped back out to disk anyway) but if you're not taking that approach there's not always a need to have a dedicated one at a all if you're not sure of how big it should be.
|