Appropriate size for / root partition

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.

Firstly, there is no /system partition. The thing you are referring to is better called as the root directory /, which is where the "root" of the file system is kept. As you have already seen, keeping a separate /home partition allows you to limit the size of / by quite a bit.

I've found 15gb an adequate amount for most installations. You could go lower, but the challenge I've most often faced is the apt archives taking up space while upgrading. If you install lots of "large" software, or keep things in other places such as /var (web server files or databases for eg), you could either keep /var on a separate partition as well, or increase the size appropriately.