The problem in that thread was caused by building ports, which will fill up /usr. If you're going to build ports, I'd recommend changing the working dirs[0] to a different partition (I use /home).
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).