iTunes stores the backups it makes in the following folder: ~/Library/Application Support/MobileSync/Backup
Note that this is NOT the library at the root of the Hard Drive but is the library that is in your Home folder.
Get there by going to the GO menu in Finder. If the library is not visible, access the GO menu while holding down the Option key.
Once there, select one of your backups and Right Click to Get Info. Now that you know the size of the backup, you should be able to figure out how much HD space you will need for additional iDevices.
Is that what you needed?
Interesting, useless (or maybe not) realization on my part. I just discovered about my 32 gig iPod Touch.
I had about 10 gigs worth of avi movies stored on there, that I had been playing with luberplayer. I threw them away, and when I re-synced my iPod to my Mac, it freed up 10 gigs on my computers hard drive.
Well "duh"... when I back my iPod up, it backs everything up.
Still, this made me think. I ask myself, if I had a 64 gig iPod with non music files on it, then wouldn't I need to to keep 64 gigs free on my hard drive? If I had multiple IOS devices synced to one computer, then I would need to keep the equivalent amount of hard drive space open to accommodate backing them up.
I'm thinking about adding an iPad or two, to my collection of apple devices. If I don't back up to the iCloud, how much space might I expect to be occupied on my main computer? Do I just need to keep room for the files that are not duplicates of songs and movies already stored in my iTunes? Or does and IOS device keep a certain amount of cache on my computers' hard drive that may, or may not actually contain data?
I guess this is the kind of stuff IT people deal with all the time. It's new to me though.
macbook pro - snow leopard
iPod Touch 4G - IOS5