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.
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.