The default GUI install of ESX4 makes the /tmp partition 1GB and even then it is only categorized as optional. I’ve been asked several times why you’d want to make /tmp any bigger. If it fills up you just clear it out, right?
Well here’s a good reason. It seems that VUM (vCenter Update Manager) uses /tmp. When you stage updates, VUM copies all the patches to the folder /tmp/updatecache. It does the sensible thing and checks that there is enough space first, but if it can’t then it tries to create a ramdisk. I don’t think I’m that keen on my server’s ram being tied up with patches. Sometimes you might want to stage the patches days in advance of an outage. I’d hope that the ESX is clever enough to dump the ramdisk if there was any sort of memory contention, but still.
Anyway, with ESX3 I know the patches could accumulate to quite a size (a couple of GBs if you left them a few months). I hear ESX4 is better in this regard, however I would suggest keeping at least 2GB for /tmp during the install.
VUM isn’t a crucial service. You can always manually copy patches to a different partition, but VUM (especially the new staging feature) is a real time-saver so I know I’ll be making sure there is plenty of space in /tmp.