Your first reference mentions the idea of leaving a part of the hard drive unformatted to allow for making new versions of partitions if necessary - I might try that as I keep little user data on this machine.
Because of how the auto-partitioner decides the sizes, a 120GB disk should get the same partition sizes as a 256GB disk (the 256GB will just get more /home space). My 256GB disk has 2G each for /usr /usr/obj and /usr/src, and with that I can build kernel, userland and xenocara with no problems (unless I've filled up /usr by building Firefox from ports).
Diverting ports builds to /home is a hack that works around the wrong choice made during install (which will invariably happen when you first start out, that's ok -- be prepared to reinstall with better parameters once you learn more about what you need).
I have built Iceape on gNewSense linux on an X60 and I can say that under linux you need at least 20Gb of space and 6 hours on the core-duo with 2Gb RAM.
I always plan on a throw-away install when first playing with an operating system and repeat the install when I know what the 'rules' are. I shall be encrypting my /home just for peace of mind if I leave the laptop on the bus, so that is another thing to research.