The Linux partition howto
gives some good suggestions on dividing up your partitions. It really depends greatly on what you plan to use your system for.
If you only plan to have a few users, then /home does not need to be too large. Though, if your few users want to store large mp3 collections, then perhaps it should be big.
Unless you plan on doing web hosting and putting a lot of stuff in /var, it doesn't need to be too large either. A few GB should be plenty.
/usr is typically the biggest, since it's where all your software gets installed. If you plan to install a lot of different programs, make /usr fairly large. If you only plan to install a few packages and then leave it at that, then /usr doesn't need to be so large.
You can make /tmp separate if you like, but there's really no need to. If your users plan on sharing some files, then you could set up a /pub folder or something similar for everyone to use for whatever they like, though this is fairly insecure, so don't do it if you have security concerns.
Not sure what to tell you about sftp and the other stuff, though.