What is the general rule of thumb for this?

how much space should each partition have?

-Thanks in advance

I am not sure if this helps :-

http://www.cpanel.net/install.html

This is the recommended as per that page :-

/boot 35 Meg
/usr 2048+ Meg (If you have a 60 gig drive try 4096 for /usr, 5000 if you have an 80 gig drive, etc)
/var 1500+ Meg
/tmp 1024 Meg
/ 1024 Meg
/home grow to fill disk
swap 2x memory size

Eddy,

If you wont mind my opinion, personally I would recommend,

50 MB for /boot
4GB for /usr
4GB for /var
1GB for /tmp
/home all the rest

Swap as you said, 2 x RAM size.

The idea is, log files grow too much too fast .. besides, even on a 40 GB hard disk, /home will have 25gb and if you can use 25GB in user sites you will be able to afford a 2nd or even 3rd server! ;)

For backup, I would suggest a second drive with CPanel's incremental back up on that and also a remote back up through rsync over ssh.

Good luck with it!