There is no ideal.
I have 250GBas well
Code:
Swap 4GB (i got 2GB ram)
/ 256MB
/home, everything left
/usr 10GB, can be smaller (perhaps 7GB), i'll probably resize it later, and make /bak slice :)
/var 512MB
I could live with smaller /usr, but i don't mind if it has some extra space....
My last setup used some 5-6GB /usr
Note that you may need to make workarounds for some packages to compile, if you use small /usr, but that's very easy to do..
http://daemonforums.org/showthread.php?t=1241#post8963
Edit: Forgot to mention, that i use FreeBSD 7 as my desktop OS, if you will use it as Server os, then you will need way bigger /var.... and something different probably