How big should the partition be for a dedicated OS back-up?

[MaCnation]

I am going to partition a 160 gig hard drive to use as 1. a dedicated os back-up, i.e. no videos or pictures or anything needless, and 2. storage for some media items.
How big should the partition for the os back-up area be and still allow for some expansion as the os gets larger?

[belvdr]

At a minimum 2.5 times the size of the OS, so you can make at least two full backups.

It really depends on how often the OS is updated/changed. I'd go for at least 4 times as big as the OS, maybe more.

[Tesselator]

??? I dunno where 2.5 comes from and it's almost completely useless to put two identical back-ups on one drive.

OP: You know that the Users folder, Applications folder, and the Library folder can and most often do contain images, music, video, PDFs, and other large files which can accumulate to a very large size indeed?

If you plan to sift though all that or at least keep the Users and Library folders in a separate drive, partition or excluded from the backup then all you need for the rest of the OS typically speaking is about 150 GB in Snow Leopard and 200 GB in Leopard. But it can vary by a lot depending on what you have installed, you're install habits, and whether or not you keep all the OS folders together.

I just slimmed my OS down from about 600 GB to just under 300 GB. I killed 140 GB in my Library folder so now it's only 110 GB, I deleted 250GB from my Users folder so now it's only 60 GB, and I killed about 80 GB of data from the Applications folder so now it's only 98 GB.

The rest of the SL OS folders are really tiny in comparison. Here's me current system:

/Volumes/OSX RAID0/Library 110 GB
/Volumes/OSX RAID0/Applications 100 GB
/Volumes/OSX RAID0/Users 60 GB
-----------------------------------------------
/Volumes/OSX RAID0/System 5 GB
/Volumes/OSX RAID0/Developer 4 GB
/Volumes/OSX RAID0/Moved Items 30 KB
/Volumes/OSX RAID0/boot 20 KB
/Volumes/OSX RAID0/Resources 10 KB
/Volumes/OSX RAID0/home 0 KB
/Volumes/OSX RAID0/net 0 KB

And three or four others have told me this is about right for them as well.