I'm about to re-install my system. I like to keep a separate /home and / root partitions. I've been using (K)Ubuntu for around a year now and my / partition (almost 36Gb) never climbed over ~13Gb of used space.
What would you say is the appropriate maximum size one should set a / partition to?
It is not possible to give an answer with a specific size as it varies between taste, need, system, setup and such.
If you have / and /home on same partition or on separate partitions. For someone with a typical setup where they got /home on the same partition, then they might want a larger size, while in your case you have them on separate partitions you don't need the system partition to be as large.
Someone with a large hard disk drive can be more generous when partitioning while someone with a small solid-state disk may opt for a smaller system partition.
There is no optimal figure, so since 36 gigabyte seem to have worked good for you, I think you should continue using that.
