If all your stuff is on Drive C, then a partition a bit larger than the "Used" space on C will be enough... for now.
But I'm with mixturikal about FAT32, and also about the new superlarge backup drives.
If you have more than one hard drive in your system then you may find the System Image software will decide it wants to include some random drives. Mine insists on doing C (correct), I (internet downloads and junk) and S (the hidden anti-burglar backup). It ignores drive D - my documents drive. Everything is backed up weekly onto a series of external drives anyway, so all I want in the System Image is the operating system on drive C. But since it saves to a 2TB external drive, I don't much care