Checkout this O'Reilly site
for a good general explanation of the Linux filesystem layout. The site also talks about what to allocate to each high level directory in the structure, but those guidelines are by no means mandatory.
If you are going to separate the directories under / into their own partitions, just remember what each directory is for and size based on your best guess. If you load a lot of software, then maybe /usr, or even /opt, may need to be big. If you download a lot of tunes or have a lot of documents, then maybe /home needs to be large.
On my present setup I have everything, except swap, under /. I have a new hard drive that I will install soon and will probably separate out /var, do some aggressive log rotation, and hopefully not fill it up too soon. But then again, mine is just a simple, non critical home use system.