First of all, what I mostly do. I press A for the default partitions and then raise them. Below are some default sizes who are advised.
|b||SWAP||2-3x RAM size||–||2x RAM size|
|d||/var||512MB to 4096MB||Y||5GB|
|f||/usr||Rest of disk (at least 8GB)||Y||Rest|
For the root partition it might be smart to keep track of how large the kernel is.
This size was a default of 512MB but this is too small to install 2 kernels (kernel and kernel.old).
Another thing is the /tmp partition, this defaults to 512MB, raise it to 1GB (for tar / gzip etc)