/ (your root partition) should be at least 6 Gig, I prefer 10 Gig
swap should be at least double your memory, up to 2 Gig. Beyond 2 Gig it's usually a waste.
/var should be used if you are installing a server. It's size would vary depending on the type of server and the amount of traffic you expect.
/home Not needed on most servers. Good if you like to reinstall your system without messing with your home files.
All other patitioning judgements are on your own.