If you have an iMac with a 320gb drive inside, and if about 170gb is used
.
You could get away with a partition of, say, 180gb or so -- but that doesn't leave room for "future growth" on your internal drive. Not the best solution.
You'd probably do better by creating a backup partition of 300gb or so. Thus your backup clone can "grow" as your internal drive does.
IMPORTANT:
If you decide to create a partition on an existing drive to become your backup clone, I STRONGLY SUGGEST that you make it the "first" partition on the drive. If you maintain a Time Machine backup, put them on the "second" partition (or third, etc.). Keep the partitions that you want to be bootable "towards the top". Things just seem to work a little better that way.
You can use either CarbonCopyCloner or SuperDuper to create your clones.
Don't be intimidated by any of this. Creating that bootable backup could prove to be a lifesaver if you ever have an "I can't boot" situation with your internal drive. Just connect the backup, and you'll be booted and running again in the matter of a minute or two.