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How large should I make root, home, and swap partitions?
When I'm repartitioning my hard disk, how big should the root partition be for flawless functioning?
Even after 6-8 months after installing (v. 10.10), system files on the root partition occupy just 5.7 Gb.
It depends on what you do with your computer. When you install software, it will be installed in the root partition. It's a very personal thing. For example, I use about 15gb. If you can, with the hard disks that we have today, I suggest you to create a partition about 20gb for your root.
To continue with eNry's answer, in my case I'm using 60Gb. I know that is A LOT, but I tend to install a lot of software, including tools for development, audio editing and academic work. Last time I had a root part with 40Gb that got almost full... and my data was in other partition!
I use 15gb for my root partition, and 2.8gb for swap space, it is entirely up to you though and as mentioned above, a, very personal, taste....